Developing software has as side effect the creation of bugs that compromise the quality of it and add to the total cost of the solution…
http://www.devtopics.com/20-famous-software-disasters Here you can get a feeling of what can be caused by a bug:
There is no software that is completely bug-free. The number of them is decreasing as a project matures and they are found and fixed. Yet, as the platform expands to cover additional functionality they are reintroduced or they become apparent just because a specific combination of events creates a new execution path that was never before followed.
A project manager is always facing the dilemma if he should go ahead and ship his code containing some known bugs or he should extend his QA phase until they are all fixed. The industry has taught us that the most successful projects are those shipped relatively early followed by frequent patches, updates and new versions than those who strive for perfection, usually missing deadlines and subsequently allowing a competitor to expand his market share.
Bug discovery, tracking and fixing is a cycle that has to be taken very seriously and holds a central role in the success of a project. Since bugs are so common in software development and their impact is so critical, developers have used various methods of working around them, varying from simple text files or spreadsheets to sophisticated tools developed exactly for them.
Companies like Jira, Fogbugz, youtrack, seapine, extraview, websina and many more specialize in the development of bug tracking software having thousands of development shops basing their operational models on one of them:
http://www.atlassian.com/software/jira/ http://www.fogcreek.com/fogbugz/ http://www.jetbrains.com/youtrack/ http://www.seapine.com/index.html http://www.extraview.com/site/ http://www.websina.com/index.html