If you don't know about Mac, maybe it would be wiser to withhold comment on details of its keyboard usage?
The Mac keyboard layout is horrendously broken in all regards. Basically, there is no Home/End key. Well, actually, there are
Home/End keys on some Apple keyboards. However, Apple's keyboard guidelines specify that these keys must be useless. (They move scrollbars to start and end, similar to Ctrl-Home/End on a PC, except that they also tend not to move the cursor position at all, only the view, which moves them firmly into chocolate fireguard territory.)
So, they've broken Home/End. But Home/End are essential for all high-level keyboard users. Ooops! What now?
You guessed it - Command-Left and Command-Right do what Home/End ought to do.
(I think Ctrl-Left and Ctrl-Right still work to go back/forward a word same as on Windows, not using a Mac right now to check this. But Command-Left/Right are definitely Home/End.)
To add to the comedy value, many Mac applications (especially cross-platform ones) provide options or default behaviour to make Home and End work sensibly, in order to work around the operating system's insanely awful default behaviour. In most of these applications, Command-Left and Command-Right also continue to work, but in some, they don't.
(As it happens, I do seriously think that these keyboard shortcut issues are the single worst thing about OS X.)
Anyhow, leaving that aside - yes this same thing has happened to me several times. Command-Left ought to go to start of line, but (in Firefox on a Mac) it tends to go Back and potentially lose all your work. I think this is a bug in Firefox really, they should get rid of that shortcut since it overlays common keyboard behaviour. Don't know what Safari does, I don't use it.
The good news is that TinyMCE isn't broken in this manner* and works with ordinary form data - you can go back and forwards with TinyMCE without losing everything. We use TinyMCE at the OU now and I think this is safe.
* TinyMCE is broken in other ways, the key one being that it requires about a 653 MB download