Time travel debugging is soooo cool and I wish it was more widespread (of course I'm aware it's very difficult to implement and has unavoidable overhead). Imagine if an automated error report from a user of your app included not only a stack trace (or whatever have you), but the execution steps leading up to the error.

  This is NOT a production C interpreter:
  
    Subset of C (no preprocessor, typedefs, unions, enums, function pointers)
    No optimization or JIT compilation
    Limited standard library (only built-in functions)
    No file I/O or external system interaction
    Fixed memory sizes

Still probably a good learning tool, but that does limit it to toy/teaching codebases, not real existing programs.