ok... so now I've learned it the hard way:
I wanted to replace a suspected-faulty "/lib/libc.so.6" with a different one, knowing to work fine.
the result was that sshd immediately disconnected me, because the whole machine froze -
I can't even login now anymore (not even locally), so I'll have to fire up something like knoppix to copy "/lib/libc.so.6.org" back to where it belongs.
great, huh?