The sort of behavior often exhibited by many, many programs. No one can tell what happened, why it happened and how to reproduce it. Just the sort of bug that everyone hates.
Sometimes the bugs can be explained and fixed later. This often doesn't happen - some bugs just can't be reproduced (see also Lubarsky's Law of Cybernetic Entomology).
Sometimes the bugs cause just plain odd, weird or sometimes even completely hilarious results.
- In one graphic hack I'm working on, the picture suddently went upside down. (This was fixed later. The example of OpenGL's gluOrtho2d() in the book specified kind of strange coordinate system.)
- Today the URL that was selected on Netscape's Location bar scrolled back and forth character at time. (God knows why, but as usual, He doesn't tell.)
- Steve Jackson (not the SJG one) told about a bug in Peter Molyneux's upcoming game Black & White: a pig somehow ate an ape.
Not to even mention some of the most hilarious bugs in Infocom games... "Bartender, give me a drink." "Sorry, I'm hired to mix drinks and that's all." =)