After beating the boss of a chapter it is possible that Bandage Girl becomes glitchy at a certain chance (in that certain chapter - light or dark world doesn't matter). Save her in this glitchy state to unlock the glitch level of that chapter. Each one brings you an achievement when you beat it. As far as I know there is only A+ or death (or I somehow managed to get an A+ on everyone of the glitch levels at first try O_o). Not getting the achievement after beating the level is just a bug I think...but I'm not sure with that...

Every time you die, there's a certain chance that Bandage Girl becomes glitchy (in that certain chapter - light or dark world doesn't matter). Save her in this glitchy state to unlock the glitch level of that chapter. Each one brings you an achievement when you beat it. As far as I know there is only A+ or death (or I somehow managed to get an A+ on everyone of the glitch levels at first try O_o). Not getting the achievement after beating the level is just a bug I think...but I'm not sure with that...