Originally Posted by meme
Full HTML Tidy does change your code some, and if you had certain errors in it it could delete parts - though rare.
Not that rare in my experience. I class it as a very dangerous function, to be avoided unless you have a special reason - and I can't imagine what that reason could be! The chance of data loss is a pretty negative feature.
On a related point, we still badly need a 'save it as it is - I'll fix the errors later' option. Sometimes an editing session has to end NOW.