It has been said that all programs contain at least one bug, and all programs have a superfluous line. Of course, this logic will lead to a program with one line with a bug in it, but you get the idea. It's basically saying that even the best software can still be improved upon.
Then there's also the Murphy's Law-style comment, 'There's always one more bug'
I have a load of Murphy's Law-type stuff on a poster