"Just when you thought it was safe" to do this began as ad copy for "Jaws 2"
[DOC] [TLE] Amazon.ca:Customer Reviews: Jaws 2 [Blu-ray] (Bilingual) [PAR] Jaws 2 can't touch the original, but it's far better than the countless imitations ... 
And since teen slasher flicks began popping out around everywhere in '78 .... 
Just when you thought it was safe.