It looks like some code was copy and pasted...

...and sfi was supposed to be replaced by ffi throughout the block,
but wasn't. For gcc this produces a scoping error which points out the
problem. Windows compilers seem to have different scoping rules, so
this may have compiled for whoever submitted the patch.

src/game_events.cpp

@@ -2240,7 +2240,7 @@ static bool process_event(event_handler& handler, const queued_event& ev)
 		if(unit2 != units->end() && game_events::matches_special_filter(ev.data.child("second"),*ffi)) {
 			special_matches = true;
 		}
-		if(!sfi->empty()) {
+		if(!ffi->empty()) {
 			filtered_unit2 = true;
 		}
 	}