Is there any harmful commands using GIT and HG

I'm teaching HG to my students, as it is a good playskool DVCS (not powerful as GIT but simple to start working with trivial concepts). I use HG because it seems very difficult to destroy previous entries (with the exception of hg rollback), so you have always a chance to get back on train without destroying important data. But i was recently wondering:

• is it true ?
• does GIT offers the same protection (i read somewhere about rebase option which can be very dangerous)