Well, apparently a large part of the episode is a flashback to a relationship Jack had with a guy in the 1920's. Everyone's assumed this was in the 1920's when Jack was living out his time on earth at Torchwood, so after the Victorian Lesbians recruit him, only, he's wearing his RAF coat from WW2, which would imply it's after WW2 when he aquired it, AND he calls himself a "fixed point in time", which is something he doesn't know about himself until after he goes back to see the Doctor at the end of TW S2, DW S3. All of which implies he must be time travelling back to the twenties in order to have this relationship, which either puts it between TW S2 and CoE, which would be weird because he's allegedly with Ianto at that point, and has no reason to be time travelling around, or it puts it post CoE, which, again, doesn't make any sense because he claims to have been keeping an eye on Gwen all the time post CoE and anyway BOTH of those options require a working vortex manipulator and we all know that was disabled at the end of DW S3. So, in conclusion, not a single one of those possibilities is canonically possible and Jane Espenson is now tweeting around going OH MY BAD (she literally said that) and trying to come up with a plausible explanation for the whole thing and failing miserably. It's hella funny.