I’m sure it happened to you too.
Just remember. That horrible moment when you time-travelled using undo - to copy and paste that line you deleted twenty minutes ago but for some reason you really, really, need it now - and then you hit a key and insert a letter. And just like that, your way back is gone.
The only appropriate reaction to that is either eating your computer or turning into a hulk and going on a neighbourhood-destroying rampage.
Yea, as a matter of fact, that has happened to me. Great Vim tip, especially combined with saving work when switching away from Vim.