The built-in openssl is version 0.9.8-something-or-other, which is pretty badly out of date. Today I tried to upgrade that to 1.0.1. When I did, my laptop suddenly became very, very unhappy. Mail stopped working. Safari stopped working.
OK, no problem, I thought, I'll just fire up Time Machine and revert the changes.
Except that Time Machine wasn't working either! Not only was it not working, it was not working in a particularly nasty way: I could get in to Time Machine, but it didn't actually work. And once I was in, I couldn't get out. The only way to escape was to power down my machine.
Then it wouldn't reboot. I nearly had a heart attack.
I had to boot from the recovery partition and revert the changes from a SuperDuper snapshot I had taken a few weeks before (thank God I had that!)
I had no idea that openssl was woven so deeply into the fabric of OS X that changing it makes the machine unbootable. In fact, even now I cannot think what mechanism could cause booting to fail. It didn't even cause a kernel panic, it just never got past the power-up spinner.