In past days I wrote about reporting bugs.
Where do bugs born ? Who knows! :)
Bugs happen for several reasons: quick fix, stress, “don’t know what I’m doing“syndrome, and lots more.
The Pragmatic Programmer talks about Programming by Coincidence:
This “programming life-style” affect the basic idea of business; it doesn’t let you focus your efforts on solving business problems, but on bug hunting and regression testing.
Maybe this is not your situation but … who knows.