Okay, so... I used the Portal 2 elevator tutorial on the mapping wiki, but when I try to run my map, it tells me that there's a leak. When I load the pointfile, it says the leak is on the elevator at the bottom of the shaft, leaking downwards, but the tutorial never said anything about sealing up the bottom of the elevator shaft. I get the feeling I've done SOMETHING wrong, but I don't know what...

I've included my VMF. I hope someone can help me with this.

You did a step wrong. The wiki tutorial says:

Quote:
1) Select the entity tool, create a func_instance, and give it the fixup name "Arrival_Elevator" and the VMF file name "instances/turbine_elevator/arrival_elevator_a4_base.vmf"

2) To make things simpler, and to not have to create two func_instances and find the second one through all this mess, simply Shift+drag the "Arrival_Elevator" one unit in any direction to copy the first func_instance, and quickly put the copy exactly back on top of the original.

3) Double-click either one and change the name to "Arrival_Logic" with the VMF file name "instances/turbine_elevator/arrival_elevator_a4_logic.vmf"

It looks like, whilst performing step 3, you have renamed both instances, instead of just one of them, which has created two sets of logic entities, and no base brushwork. To fix it, select one of the func_instances and set it's vmf filename back to "instances/turbine_elevator/arrival_elevator_a4_base.vmf"

Ah, so that's what happened. I honestly had no idea. I did what the tutorial said and did shift-drag to clone the instance, but for some reason, it linked the two together- if I changed one, the other changed with it. I had to delete one and redo the cloning to get it to work. Thanks!