Disconnect by Server: Nope!

I was playing on my Bukkit Minecraft server, testing a new plugin for sitting in chairs. I right click on the chair (A stair block), and am teleported to the stair block. I right click another block (That's how you get out) and am kicked, with the server telling my "Disconnected by Server: Nope!" When getting out, I'm supposed to teleport to the block that I right-clicked on, but instead am kicked. I don't have any anti-cheats/anti-hack plugins.

Could somebody explain to me what is happening, and how to prevent/fix it?

This is probably related to a known bug that is being worked on. Chairz FAQ:

Sometimes I get kicked off the server. It says "Nope". Why does this happen?

• This can sometimes happen when you click the chair twice. You only need to click it once. This will be fixed too, in a later release.

Clicking on another chair while already in a chair or accidentally clicking the same chair you're in due to hitbox weirdness (as you may have done) would be exactly the same as far as the plugin code is concerned as clicking on a chair twice (as the FAQ mentions), and would trigger the same bug.

I would say that testing this new plugin before committing to it was wise! The fix appears to be to wait for the author to addresses the root cause of this bug. Reporting it with an exact, reproducible description of how you triggered the server kick might be helpful, especially if it's not a double-click or something similar, since you may have found a new way to trigger this bug.