There were fifty of them, Edd, I swear! We tried to hold them back but they just kept coming! We did everything we could, but...
We're really, really sorry.
The stupid thing got me by surprise right as I came through and killed me, and then a second one got Hula when he came to investigate. Three of the pistons still work but the odd one in that screenshot is stuck open, and the outside lever is gone. I reckon the piston could be fixed quite easily but we didn't have any redstone on us; I've no idea how much damage was done to the lever. It was lucky Hula was quicker on his toes than I; he lured the second one away from the circuitry before it exploded, or else it might have been much worse.
In light of this, I think it wise to stick a sign on the inside of the gates, warning people to check for creepers as they go through, to discourage people from being as foolish as I was.