I remember doing this mistake back in college. Was required to program and demo the B-Tree backend for a mini database. Things were very well tested for tons of random sets of data but while waiting for the my turn to demo, added one line of debug print on an &quot;if&quot; statement which caused the now dangling statement to be always executed. This caused a subtle bug that the professor caught in his testing. Sucked because I had everything correct up to that point. Learned my lesson about last might changes and coding style at that point.