When in the city when you have to lower the walkway to activate the horde.

Instead of lowering the walk way simply go up to the mini-gun (turret) and melee it about 10-15 times, striking it and then hop on it.

This will send you flying over the blockade.

This can be used on any turret. It is an easter egg set up by the makers of L4D.

When the turret is "charged up' you can then use it to jump great distances.

[unconfirmed]