1. Any code that uses disk files for temporary variable storage. It means speed is throttled by disk IO, which makes delayed write required, which threatens the entire system file system. 2. Yes: "Option base 1" is such a bad idea. 3. Any do-nothing wait loop without an abort mechanism: the HP/RMB BASIC "WAIT {seconds}" would stop the whole machine, including the display and IO while it did nothing. 4. Any compiler that allows uninitialized variables. Random values cause unpredictable behavior/crashes. 5. Any (real time/ ISR) code that will stack overflow if the CPU is too slow. ...etc