Early on in my Linux-using-life I made the mistake of deleting /etc. That was a learning experience like no other :)
Probably the easiest way to experiment these days is to create a VM and make snapshot, then start knocking down walls, just to see when and where the house collapses. Then revert and try something new.
There could be a competition!
So I ran two simple commands:
cd /tmp
rm -fr .*
> When a pattern is used for pathname expansion, the character ``.'' at the start of a name or immediately following a slash must be matched explicitly, unless the shell option dotglob is set. The filenames ``.'' and ``..'' must always be matched explicitly, even if dotglob is set.