Exceptions and Modula-2?
In my humble opinion this is one of the nice concepts introduced with ISO Modula-2 (not really ISO: I had to learn that Niklaus Wirth already had an implementation of exceptions on the PDP-11 version of the Modula-2 compiler – that was 1979! Thank you people for pointing this out in comp.lang.modula2;). Exceptions are a really powerful way of handling exceptional situations that are errors of your program.