A fatal error can be a lot of things. It's like says "I just got an error in Windows, what's wrong?" - Hard to tell
I got an error once, because I had a trigger adding a hero ability to a normal unit. Apparently Wc3 doesn't like that
If it does it immediately on loading your map, you probably imported a corrupted model / skin. The only way I know of of fixing this is loading a save from before you imported that model / skin and never import that model / skin now.
I prefer not to get a copy of the map, and check the error. I prefer giving suggestions about possible solutions, that way you learn much more
I don't get anything special from the error message itself, but usually you can determine around where the bug lies.
During Loadingscreen: Imported object corrupted, most likely.
During Map: Depends on what triggers are running.
If you have a trigger that spawns 1 footman when you type "-gimmeh!", and the game conveniently crashes when you type the command, you would know where to look.
It gets easier if you only have 1-50 triggers, as there aren't a LOT running at once, and further more if you know exactly how and why your triggers do what they do... It's much easier for the mapmaker to bugsearch his/her own map, than for other people to read through all the coding...
2. loading screen corrupted or bad format (this usually happens instantly, you cant even get into the game)
3. From what i heard, Moving a unit into map boundaries causes fatal error too. Although i never tested this before.
4. Negative or "forced" entered (shift+enter) values in spells in the object editor, i experienced this before, happened to me when i "forced entered" the chain lighting number of bounces to over 32.
I don't think I'm dealing with quite the same pressures though, at the very least its a significantly smaller system. For the time being I'm just going to put together a quick scrubby box though and hope it works good enough to not make my house toxic
I do know this - xenforo dropped the ball by not keeping the vbulletin reputation comments as a feature. The loss of the Reputation comments data when we switched to Xenforo really was the death knell for the site when it came to all the users that left. I know I missed it so much and I got way less interested in the site when that feature was gone and I run the site.