Infinite Robin / Nightwing Glitch
As if one Robert wasn't enough, now we have an army of Boy Wonders...
To activate this glitch, unlock the Upgrade Red Brick for "Decoy" ability for Tech Suit. Equipt Robin or Nightwing with the tech suit and place a decoy. Before the decoy explodes, you can swap into it. just make sure you don't explode near the others or you'll have to start over (jumping off the platform is an easy way to avoid mass destruction)
Unfortunately this glitch only works in the Bat-Cave, so I wouldn't count on an army to take on Joker or anything.