What is a computer bug?

In 1947 Harvard University was operating a room-sized computer called the Mark II.

  • mechanical relays
  • glowing vacuum tubes
  • technicians program the computer by reconfiguring it
  • Technicians had to change the occasional vacuum tube.

A moth flew into the computer and was zapped by the high voltage when it landed on a relay.

Hence, the first computer bug!

  • I am not making this up 🙂

Bugs a.k.a. …

  1. Defect
  2. Fault
  3. Problem
  4. Error
  5. Incident
  6. Anomaly
  7. Variance
  8. Failure
  9. Inconsistency
  10. Product Anomaly
  11. Product Incidence
  12. Feature

Defective Software

We develop programs that contain defects

How many?
What kind?

Hard to predict the future, however, …it is highly likely, that the software we (including you!) Will develop in the future will not be significantly better.

Sources of Problems

Requirements Definition:
Erroneous, incomplete, inconsistent requirements.

Fundamental design flaws in the software.

Mistakes in chip fabrication, wiring, programming faults, malicious code.

Support Systems:
Poor programming languages, faulty compilers, and debuggers, misleading development tools.

Inadequate Testing of Software:
Incomplete testing, poor verification, mistakes in debugging.

Sloppy redevelopment or maintenance, an introduction of new flaws in attempts to fix old flaws, incremental escalation to inordinate complexity.

Adverse Effects of Faulty Software

Loss or corruption of communication media, non-delivery of data.

Space Applications:
Lost lives launch delays.

Defense and Warfare:
Misidentification of friend or foe.

Deaths, delays, sudden acceleration, inability to break.

Safety-critical Applications:
Death, injuries.

Electric Power:
Death, injuries, power outages, long-term health hazards (radiation).

Money Management:
Fraud, violation of privacy, the shutdown of stock exchanges and banks, negative interest rates.

Control of Elections:
Wrong results (intentional or non-intentional).

Control of Jails:
Technology-aided escape attempts and successes, accidental release of inmates, failures in software controlled locks.

Law Enforcement:
False arrests and imprisonments.

Bug in Space Code

  1. Project Mercury’s FORTRAN code had the following fault:
    DO I=1.10 instead of … DO I=1,10
  2. The fault was discovered in an analysis of why the software did not seem to generate results that were sufficiently accurate.
  3. The erroneous 1.10 would cause the loop to be executed exactly once!

Military Aviation Problems

  • An F-18 crashed because of a missing exception condition: if … then … without the else clause that was thought could not possibly arise.
  • In a simulation, an F-16 program bug caused the virtual plane to flip over whenever it crossed the equator, as a result of a missing minus sign to indicate south latitude.

Year Ambiguities

  • In 1992, Mary Bandar received an invitation to attend a kindergarten in Winona, Minnesota, along with others born in ’88.
  • Mary was 104 years old at the time.
  • Mr. Blodgett’s auto insurance rate tripled when he turned 101.
  • He was the computer program’s first driver over 100, and his age was interpreted as 1.
  • This is a double blunder because the program’s definition of a teenager is someone under 20!

Dates, Times, and Integers

  • The number 32,768 = has caused all sorts of grief from the overflowing of 16-bit words.
  • A Washington D.C. hospital computer system collapsed on September 19, 1989, days after January 1, 1900, forcing a lengthy period of manual operation.
  • COBOL uses a two-character date field …
  • The Linux term program, which allows simultaneous multiple sessions over a single modem dialup connection, died worldwide on October 26, 1993.
  • The cause was the overflow of an int variable that should have been defined as an unsigned int.

Shaky Math

  • In the US, five nuclear power plants were shut down in 1979 because of a program fault in a simulation program used to design a nuclear reactor to withstand earthquakes.
  • This program fault was, unfortunately, discovered after the power plants were built!
  • Apparently, the arithmetic sum of a set of numbers was taken, instead of the sum of the absolute values.
  • The five reactors would probably not have survived an earthquake that was as strong as the strongest earthquake ever recorded in the area.

Therac-25 Radiation “Therapy”

  • In Texas, 1986, a man received between 16,500-25,000 rads in less than 1 sec, over an area of about 1 cm.
  • He lost his left arm and died of complications 5 months later.
  • In Texas, 1986, a man received at least 4,000 rads in the right temporal lobe of his brain. The patient eventually died as a result of the overdose.
  • In Washington, 1987, a patient received 8,000-10,000 rads instead of the prescribed 86 rads.
  • The patient died of complications of the radiation overdose.

AT&T Bug: Hello? … Hello?

  • In mid-December 1989, AT&T installed new software in 114 electronic switching systems.
  • On January 15, 1990, 5 million calls were blocked during a 9 hour period nationwide.
  • The bug was traced to a C program that contained a break statement within a switch clause nested within a loop.
  • The switch clause was part of a loop. Initially, the loop contained only if clauses with break statements to exit the loop.
  • When the control logic became complicated, a switch clause was added to improve the readability of the code.

Bank Generosity

  • A Norwegian bank ATM consistently dispersed 10 times the amount required.
  • Many people joyously joined the queues as the word spread.
  • A software flaw caused a UK bank to duplicate every transfer payment request for half an hour. The bank lost 2 billion British pounds!
  • The bank eventually recovered the funds but lost half a million pounds in potential interest.

Making Rupee!

  • An Australian man purchased $104,500 worth of Sri Lankan Rupees.
  • The next day he sold the Rupees to another bank for $440,258.
  • The first bank’s software had displayed a bogus exchange rate in the Rupee position!
  • A judge ruled that the man had acted without intended fraud and could keep the extra $335,758!

Bug in BoNY Software

  • The Bank of New York (BoNY) had a $32 billion overdraft as the result of a 16-bit integer counter that went unchecked.
  • BoNY was unable to process the incoming credits from security transfers, while the NY Federal Reserve automatically debited BoNY’s cash account.
  • BoNY had to borrow $24 billion to cover itself for 1 day until the software was fixed.
  • The bug cost BoNY $5 million in interest payments.


  • Have you heard of other software bugs?
    • In the media?
    • From personal experience?
  • Does this embarrass you as a future software engineer?


if you can’t say it, you can’t do it

  • You have to know what your product is before you can say if it has a bug.
  • A specification defines the product being created and includes:
    • Functional requirements that describe the features the product will support. E.g., on a word processor
      • Save, print, check to spell, change the font,
    • Non-functional requirements are constraints on the product. E.g,
      • Security, reliability, user-friendliness, platform

A software bug occurs when at least one of these rules is true

  • The software does not do something that the specification says it should do.
  • The software does something that the specification says it should not do.
  • The software does something that the specification does not mention.
  • The software does not do something that the product specification does not mention but should.
  • The software is difficult to understand, hard to use, slow

Most bugs are not because of mistakes in the code

  • Specification (~= 55%)
  • Design (~= 25%)
  • Code (~= 15%)
  • Other (~= 5%)

Relative cost of bugs “bugs found later cost more to fix”

  • The cost to fix a bug increases exponentially (10x)
    • i.e., it increases tenfold as time increases
  • E.g., a bug found during specification costs $1 to fix.
  • if found in design cost is $10
  • if found in code cost is $100
  • if found in released software cost is $1000

Bug-Free Software

  • The software is in the news for the wrong reason
  • The security breach, Mars Lander lost, hackers getting credit card information, etc.
  • Why can’t software engineers develop software that just works?
  • As software gets more features and supports more platforms it becomes increasingly difficult to make it create bug-free.


  • Do you think bug free software is unattainable?
    • Are their technical barriers that make this impossible?
    • Is it just a question of time before we can do this?
    • Are we missing technology or processes?

The goal of a software tester

  • to find bugs
  • as early in the software development processes as possible
  • and make sure they get fixed.

Be careful not to get caught in the dangerous spiral of unattainable perfection.

What to look for when interviewing someone for the position of a software tester

  1. Are they explorers?
  2. Are they troubleshooters?
  3. Are they relentless?
  4. Are they creative?
  5. Are they perfectionists (within reason)?
  6. Do they exercise good judgment?
  7. Are they tactful and diplomatic?
  8. Are they persuasive?

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