ISO demands that throw/1 make a copy of Exception, walk up the stack to a catch/3 call, backtrack and try to unify the copy of Exception with Catcher. SWI-Prolog delays making a copy of Exception and backtracking until it actually finds a matching catch/3 goal. The advantage is that we can start the debugger at the first possible location while preserving the entire exception context if there is no matching catch/3 goal. This approach can lead to different behaviour if Goal and Catcher of catch/3 call shared variables. We assume this to be highly unlikely and could not think of a scenario where this is useful.47I'd like to acknowledge Bart Demoen for his clarifications on these matters.