Commonly used definitions

bitrot, n.:

Also bitdecay, coderot. Hypothetical disease the existence of which has been deduced from the observation that unused programs or features will often stop working after sufficient time has passed, even if ``nothing has changed.'' The theory explains that bits decay as if they were radioactive. As time passes, the contents of a file or the code in a program will become increasingly garbled.
http://www.bitrot.net/
Over time, your code will be subject to more and more sets of possible runtime paths. It is only a matter of time that bugs in your code will begin to exhibit themselves more and more.

Heisenbug, n.:

A bug whose presence is affected by observing it (or by the attempt to do so). An example of this would be
assert(x=5);
This is a bug that is effected by whether you disable/enable the assert macro.

petbug, n.:

A bug that you have allowed to live long enough to give it a name.