Hidden button in Ultimate Marvel vs. Capcom 3 performs an instant character swap for one bar, videos show resulting glitches from use
It enables you to swap your first and second characters instantly, costing you one bar of super meter.
This feature was actually mapped to a single button which is otherwise inaccessible — unless you edit your save file — but the fact it exists, and at least was partially balanced (since it costs a meter) is fairly interesting. Here's a quick glimpse of the feature in action.

Click image for animated sequence
Pat728 put together a couple of great videos showing how this feature worked, and even a combo/glitch video illustrating some of the wild possibilities it would have opened up. You can find them below, along with instructions on how it's done.
How to access the hidden button feature
1. Go to the training mode.
2. Then go to button configuration option.
3. Decide which button you want to set this feature.
4. The button you want to use it change to S1 Recording.
5. Open the savegame with PS3 Bruteforce.
6. Go to hex edit.
7. Search for the P1 hex area which is located in 811 - 83C
8. Search for the value 20
9. Replace it with the value 80.
Note: This does not work online.
Instructions from TheSkyrae, who made the initial discovery of this hidden fast swap feature. Mentioned by Soriku and PG|Filipino Champ.