I always go in with good intentions, but then the code is already written, and it works, and re writing it would be boring...

Edit: Now that I think about it, it isn't the re-writing I mind so much, it is the prospect of re-debugging

Oh so this happens to other people too! I'm kind of relieved.

That requires managing expectations. Sometimes the way to do that is not tell anyone you have anything working, just "Yes, that should be possible".