I found this tends to happen when you have stuff going on under or around the sign, like very fast redstone circuits.
one time i was writing a sign with this guy in a room that was bugging me, and i was writing and it deleted and all that was on the sign said "shut up" this was pretty scary, but i bet the guy was hacking, his name was like bubba or bubby something.
@Meincravta if you find someone hacking, or if you suspect it, mash the F2 key and send a moderator the screenshot and note the time/server information. They will be able to review the situation and place the appropriate ban on the individual if it is found that they were, in fact, hacking. Blessings!
FYI, my Mac computer is a P.O.S. It doesn't even have a right click, making crafting almost impossible, all the F1-12 buttons have a use, like f2 is brightness, even though this is like the newest model. I hate my computer !!!!
Use function-f2. And as for having no right click, get a new mouse. I play on a mac too, and it works just fine for me. P.S. Function is that little key at the bottom left of your keyboard that says "fn" on it.