What happend here? Bug?

Can someone explain me what happend here?
http://nl.lichess.org/vFaQsdFa/black#4

Move 4 to 5. How can he take my piece here?

It's no a bug, it's a rule of chess.

Is name is the hold "en passant":

When you move a pion of two case, a pion opponent can eat your pion as your pion move in one case.

More information here: en.m.wikipedia.org/wiki/En_passant

This is called the "En Passant" rule.

It is the only other weird rule in chess aside from castling.

see this video for details. www.youtube.com/watch?v=tOIgnoG5YjY

:) It's called en passant - there's a thread here that describes the move. It catches a lot of people out so you're not alone.
Here's a wikipedia article on it
http://en.wikipedia.org/wiki/En_passant

In the name of god. Wow, i never heard of this! Gonna that this out.. Thanks for the info..

Blame it on the French :)