I had a coworker on one project that took input as short integer values between 0 and 32767. The values conceptually represented min and max values between 0.0 and 1.0, so one day he changed all the parameters to Float, like in hundreds of places. Since the application then wouldn't compile, he wrapped all the calls via search-and-replace so that the integer parameters were cast to Float and then clamped between 0.0 and 1.0.

Viola! the application then compiled. Unfortunately, this application controlled a baseball-throwing machine capable of 125 mph and he decided to test his new version (for the very first time) on an actual baseball field with Little Leaguers on it. The machine's first throw was supposed to be a soft ground ball to the shortstop, but was instead a 125 mph knuckleball a foot above the kid's head. This lead to the only time in my programming career when I had to physically intervene to prevent a fistfight (between the client and my manager).