In the late 1980s I fought simple profanity filters that wouldn't let you send the words wristwatch or Scunthorpe United. Impressive just how far we've come.
10 years ago at school I could not search for documentation about strings or "insert" clauses (sql) because that would trigger the firewall. Fun times.
In 2017 I couldn't get a price on anything in the Cisco Select range because clicking the link triggered a WAF on the distributor's servers.
My GF worked on an mmo's tweens. Said one experiment with filters ended with a 12 you old boy writing, I'm going to stick my giraffe up your pink bunny.