The only thing I thought of was a mod in /v messing with me or trying to tel me i have too many pistons.
First of all, this is not something the staff members would do. Some other servers may be more lax, but the staff on this server are typically harsh about this kind of action.
What it sounds like is that lag has caused a block to lag into a frame of existence and non-existence. Not sure how well I can explain, and I may get it wrong, but here goes:
Basically, sometimes, when a block gets input to be moved or is destroyed, it changes states from existing to not existing. So when you break a block (input), that block stops existing, and instead an item exists in its place. When a piston moves (from redstone input), both the piston and block don't exist where they were, but now they exist where they end up. So they stop existing at point A and start existing at point B.
What sometimes happens is that when your computer or the server lags, it causes a break in input, thus causing the block to not know whether it exists or not, where it may/may not exist, or in what form, because it didn't get the input properly. In breaking blocks, this can cause a space to be empty, but you can't place blocks on it, and if you try to walk on it you may lag and not be able to move through. With pistons, this can cause the block, usually, to disappear and have the same effect, or I guess in your case cause the piston to disappear.
Side note: this also happens with doors, sometimes. If you ever open a door, then shut it, but lag causes it to appear open, then you try to walk through, but can't, then you right click the door again and it snaps back open again, this is basically the same issue. The door didn't get the input properly, so though the game knows it's supposed to be closed, it appears to be open.
So to test this, try these things:
1. Place a block where the pistons are supposed to be. If it places, then suddenly gets removed back to your inventory, then lag caused the problem.
. . 1.a. If this happens, log off and back onto the server, or restart Minecraft. If the pistons return, your computer lagging caused the problem.
. . 1.b. If restarting Minecraft doesn't work, and placing a block in the empty space still causes it to place but suddenly get removed to your inventory, then it is likely a server-side issue, caused from when the server lagged. For this you'll have to wait for a server reset
2. If placing a block happens as usual, not causing the block to be suddenly removed to your inventory, use /inspect, hold a block, and place the block where the pistons are supposed to be. Report the name that appears in chat for griefing.
