My feet were getting cramped with standing on the bricks, and I inadvertently moved them.

The bricks came down with a rattle, and unfortunately in slipping I clutched at the trap.

This was too much for my frail prop, and the door slammed down with a great noise.
Here was a nice business for the eavesdropper! I scurried along the passage as stealthily as I could and clambered back into the store, while I heard the sound of Laputa and Henriques ferreting among the barrels.

I managed to throttle Colin and prevent him barking, but I could not get the confounded trap to close behind me.

Something had jammed in it, and it remained half a foot open.
I heard the two approaching the door, and I did the best thing that occurred to me.