He means the 'forward/back' thumb buttons on the mouse (the OS usually considers them buttons #4 and #5, with #3 being the middle button). I believe they're mapped to "View>Back" and "View>Forward" by default.
I could be wrong, of course, but my guess is that the problem might lie somewhere here. MistaSub, my advice is to try and pay extra attention when you use those buttons. Because maybe there's a Fusion bug that causes back/forward to trigger the wrong behaviour editor in some situation. Or maybe there's some situation where you're accidentally moving to a different editor than the one you think you're moving to.
Yes, you are correct about what I meant. So, this has happened twice since I posted this. And it just happened the second time, and sort of on purpose. I deleted a behavior, thought of this thread, and switched back into it from another behavior with the mouse thing I talked about (switched back to it after I had deleted it). And lo and behold, one of the behaviors in the object overwritten by the deleted one. Unless, I screwed this up, there might be a bug there. Unfortunately I am not sure if the behavior that was overwritten was the one I switched to (the one I was backing into the deleted object from), but I feel like it was another one, because I tried to test it. I don't know, I hadn't really touched this object in 2 years, and it was one of the first ones I created, and all the behaviors look pretty similar to me now on the surface. Another detail, in case it matters is I deleted all events from the behavior, then deleted the behavior, this is something I end up doing when I am refactoring.
Clarification: I mean I caused this to happen clearly, hadn't happened in a long time. At first I felt like it didn't work, because the behavior I switched from remained intact, but when I booted the game, a separate behavior in that object had indeed been overwritten. I don't know, to me that feels like either I am the biggest idiot on the planet, or there is a bug there. Both of which are completely possible.