Everyone knows about Crystal of the Elder Things and Call Ancient One, but I'm not sure if this one's been brought up before... CotET, Crowbar, and Shroud of Shadow.
http://www.arkhamhorrorwiki.com/Call_Ancient_One
http://www.arkhamhorrorwiki.com/Shroud_of_Shadow
http://www.arkhamhorrorwiki.com/Crowbar
I discard infinite sanity! I draw the entire common item deck.
Conversely, if one wishes to argue that you can only discard sanity you have, I'd argue that the same should apply to casting spells with sanity costs you can't afford. ::Sigh:: I wish FFG would just make a ruling on this saying that investigators can never spend more than their maximum sanity or stamina... Although that too would make me sad, because I would never get to say "I discard infinite sanity! I draw the entire common item deck." C'mon, you know that would sound awesome.
? I was going to argue that point for Stenun. Like, well, if you can spend more Sanity than you have, why can't you spend more money than you have
?