A pawn capturing glitch?

This has happened against me a few times now. It's a weird glitch, where I, for example, have a pawn on E7 and my opponent has a pawn on D5. I move my pawn to E5 and then he captures it and his pawn is suddenly on E6.

Here it is in action: http://en.lichess.org/d22SXNFd/black#7

On move 3 i move my pawn and on move 4 he captures it, even though it should not be possible.

Has anybody encountered this glitch? Or is it some chess rule I am not aware of?

This is not a glitch, as you say it's a chess rule called en passant. See en.wikipedia.org/wiki/En_passant.

Oh, thanks. I didn't know that.