This one requires the PC version and is more of an exploit than a bug, but there ya go:
Start the game with your main character, ideally the one you've levelled up the most. Then, Alt-Tab back to the desktop and dig inside Borderlands: TPS' folder to start the game a second time, from its executable. For better performance, dial back all of the visual bells and whistles as well as the screen resolution on that second game. Then, open the first game to LAN players and allow the second one to connect to this nonexistent network.
Once that's done, just minimize the second game's window and go back to your main character. Play as you normally would - but park one of the characters you'd like to power-level near the entrance of the zone you're in. This has the nice side-effect of cranking up the difficulty, loot levels and earned cash, so your main character also benefits from it.