Psyduck, I concede the point that the whole of events from the cave onward were unexpected. Which just further emphasized what a brilliant wizard Dumbledor was to be able to identify the liquid on sight. Of course, it also brings out just how much more of a risk it would have been to drink it, unless Severus regularly kept an antidote to this particular potion in his stores. Somehow, I find that unlikely.

But I do have one question. Were Snape and RAB (who I think we all know was Regulus Black) close associates? And if so, could RAB have conveyed some information to Snape regarding the cave and what it contained or did not contain? Could this actually be a setup on Snape's part? Any thoughts?