Here's an inexpensive way to prevent plugs from "falling" out of X10 modules and reduce the possibility of tampering. For my application, I wanted to ensure my son didn't (trivially) bypass the X10 system, so I used these plastic straps to wrap once around the plug and then the X10 unit itself. Tighten the whole arrangement and cut-off the excess and you're good to go.

This is not, of course, meant for any mission-critical or secure application.