We had a non-neglible number or clients still running on PHP 4. =( It's a dilemma indeed, because upgrading all of the servers and software is bound to break a huge portion of the websites, many of which have been loyal clients for years. (But don't have the dev experience to fix things.) Not upgrading things leads to degrading service, and upgrading things leaves a lot of clients with broken websites.

Also: hacked WordPress sites. The bane of my existence. I must've dealt with several hundred of them in my time.