I think the worse offender for me is the ever-climactic JLA "World War III". Everyone on Earth gets superpowers and flies into battle alongside a host of angels, and no one ever mentions this again? What?
See also: the end of "The Ultimates vol 2", where the entire country gets invaded and occupied for a while, and the only other mention of it is, I think, something like this in Spider-Man:
"My aunt broke her leg." "What, in that Ultimates thing?" "No."
Yes, the entire US is invaded and the president is kidnapped and one person refers to it later as "that Ultimates thing." Why do people care about Ultimatum and not this?
(However it's probably safer to just assume the Ultimate Universe ended after Ultimates vol 2, because...yeah.)