I'm still betting on something in her house but I suggested the early evening to test just in case.
Christy: Post what hardware and software, if any, you're using and we'll be better able to help you figure out what is causing the problem. Start keeping a log of the times it happens, what house / unit codes are affected, and if you just sent an X10 signal yourself.
i.e. If it is happening at a particular time each day, it might be a timed event at your house or next door causing it. If a light comes back on every time you turn it off, it could be a neighbor turning his (and your) light back on or the type of bulb you have.
BTW... It happens to the best of us from time to time even after years of debugging, filters, signal boasters, and phase couplers. Murphy gives us opportunities to tweak are systems from time to time. Sometimes macros can create and send unrecorded "ghost signals" that require placement of delays to fix. Other times a remote can get a sticky button or fall down in the couch to be triggered at the oddest times.
e.g. All my lights came on twice the other night and my activity monitor showed no "All Lights On" command at any time. I've also misplaced one of my remotes. - Hmm.. Maybe I should go lift a few cushions.