Now that I think about it, the whole PU thing could make an interesting deliberate game mechanic. The player could shift between several different versions of the stage with similar terrain, but different objects, enemies, etc. The PC could be exploring a ruined city, for instance, and come across a broken elevator. Instead of climbing up the shaft, the player could move to a PU where the elevator still works, take it up, and shift back to the original universe.

There's an indie game called eversion where you can switch the reality at certain points in the stage(indicated by a background and music change)although it's more like twisting the current universe than travelling to a paralel one,as items and enemies remain in their current state(what i'm trying to say is that a dead enemy won't respawn if you switch or "evert" although they will behave differently and items will look different).
It's free so try it out:
http://zarat.us/tra/eversion/
There's also a paid steam version with detailed backgrounds and some other options like time attack.