Flashbacks

I've been thinking about whether you can use flashbacks during a game. The answer should be "Yes" but how?

The big problem is that you don't want anything to actively contradict something in the present. 

There seem to me to be a couple of options that might work or not.

Most likely to work is the explanatory flashback. The players know what they want to explain ("Why Indy hates snakes") and they are playing it out, either because it's fun or because they want some details. The outcome is probably not in doubt (though you can set this up so the outcome determines something in the present) but we wonder about how.

In the flashback all of the players play younger versions of their characters, some do, or one do. The flashback might explain what happened when the team came together, how some of the team earned the enmity of their arch-foes, or why that regenerating character is so dangerous. (For my players: "I'm having a bad day!")

In a game with a retcon facility, this is almost a retcon.

You could make it a retcon--you might charge players if they contradict current continuity, and give them the cost back if they fix it. You'd charge them at the end of the scene, and be willing to pay them back if they fix things later. The character takes the place of the bad guys but uses a robot to replace themselves, for instance. 

If you don't know the point of the flashback, it's more likely to go wrong and contradict the current. Now, sometimes those contradictions can be useful--they give you a seed for a future story that explains the contradiction. 

Another option might be the alternate past. In this case, the scene is meant to be a cautionary tale so you want to use it in a case where you want to show a variety of options. 

The third one might be the Rasomon story. Maybe this is various characters memories or theories of how something went done--a kind of Justice League Rashomon--and the real answer is in the contradictions. You need a certain buy-in from the players so that various characters do similar things in line with the point-of-view character of the moment, but you migh be able to pull it off once. You wouldn't do it often.