I got this idea from conversations on this board in regards to Storm and Jean Grey and Mystique and Magneto.
There are certain characters in the X-Universe who interact with each other as if they've known each other for years. The first example I'm going to use is Jean Grey and Storm. Jean Grey left the X-Men and moved in with Misty Knight circa Uncanny X-Men 94, she was replaced by the Phoenix at the end of Uncanny X-Men 100. There is a back story in Classic X-Men 2 where Jean and Storm initiate a friendship. Storm grows close to the Phoenix (not Jean), Jean gets the memories of the Phoenix but as an onlooker, having not felt any of it. So in Jeans return Storm should have an affection for Jean but in all honesty Jean shouldn't know Storm very well. The same goes for Wolverine, I believe Jean and Wolverine have a Classic X-Men backstory as well so it's a little bit bizzare of her reaction to him when she returns. Particularly when you count the real Jeans reaction to Cyclops intimate moments with the Phoenix Force because she was behaving as if it was another women (which it actually was but enough of that). Obviously Jean/Storm became good friends after but at the time the scene is a bit jarring.
The next two are Mystique and Magneto, yeah Mystique and Magneto do have some history. For example, Mystique arrested Magneto in Uncanny X-Men 200. But do they actually know each other? How many conversations can you think of between the two? The last one I recall was in No More Mutants by Mike Carey which was unusual.
The next one I'll use is Dazzler and most of the X-Men. Now in fairness this is one that writers tend to get RIGHT a lot. Her not knowing Jean at all in her return to the X-Men circa X-Men - Eve of Destruction was bang on. Dazzler shouldn't know Jean cause they've never met.
The other one which is interesting was Bendis stating that Jean and Kitty knew each other. Well, truth be it's possible they could have met each other during X-Men Blue and Gold but before that Kitty heard stories of Jean but Jean didn't teach Kitty anything. In any case Kitty was less then friendly to a fake Jean Grey in Excalibur 41. So again it would make sense given that they don't know each other at all. (I'd imagine Kitty would have said the same thing to the real X-Men anyway)
Frost is the next one, Frost like Dazzler is one that is/was written as an outsider to the X-Men quite well. The ones who had animosity towards her did, the ones who didn't have any history with her still did but not to the extent of the others.
So who else can you think of has interacted with each other as if they're BFFs or more then acquaintances when they really shouldn't be.