definitely messed up with continuity for a variety of reasons (Emma Frost being in the 60's and then appearing younger in the 80's in X-Men Origins: Wolverine
, Xavier getting paralyzed far sooner than The Last Stand
depicted, the issue of Moira McTaggert, etc).
I think Singer means he's going to try and fix some of the continuity errors that have arose in the films starting with X-Men: The Last Stand
, being carried over with X-Men Origins: Wolverine
and continuing with X-Men: First Class
. At least that's the impression I get from interviews when he talks about it, even though he stresses that's not going to be the main point of Days of Future Past