Charles was going to say "Not after getting all your students reduced to their component molecules"

The Marvel Legends action figures come packaged with comics, and this latest batch included Havok. The comic included was X-Men #97, a Chris Claremont/Dave Cockrum classic, but the outside cover was from issue #58, kind of a neat one from Neal Adams. All of this, and a very nice toy, but we're talking retcons today. Sorry, Havok.

In Ed Brubaker's X-Men: Deadly Genesis, he tells of the previously unmentioned "1.5" team. After the original X-Men (minus Beast, plus Havok and Polaris) were captured by the living island Krakoa; but before recruiting the new team of Storm, Wolverine, Nightcrawler et. al; Professor X trained a group of mutants under Moira MacTaggert's care. This team, including the long-hinted third Summers brother, was more or less wiped out.

This was an interesting enough story, and a good springboard for Brubaker's upcoming Uncanny X-Men run. But it also creates some plot holes. For example, Moira is rightfully good and pissed about Xavier getting her students killed, yet she's back at Xavier's school to help out by issue #96, about three issues later. How the hell...

Moreover, in another retcon, Moira and Xavier were nearly married at some point before all this, then broke it off. So, Xavier was able to talk his ex-fiance into lending him her students for a suicide mission; then getting Moira, a Scots Guard trained, multiple-doctorate geneticist, to come back to work for him as a housekeeper. Which means:

A. Professor X has been controlling Moira's mind, on a pretty regular basis, for a very long time.

B. Moira sincerely regrets breaking up with Charles, and would do anything to get him back. Anything.

C. Professor X has the strongest pimp hand in the Marvel U.

So, if you're a writer planning to retroactively insert a big revelation into even a supporting character, that has been in a book since 1975: reread the character's appearances, with your revelation in mind. Think about whether it makes sense in that context, or if it's too impossible that your big reveal wouldn't have either come up before now, or caused that character to act differently in the past. Of course, with characters that can either control or wipe minds, you can get a little more leeway there.