Adding multiple people to PM hides the PM's title
Titles are required to send a PM. Adding users to the list (typically step 1) results in overflow covering the title box (step 2) making it look like you just need a body to fill out (step 3)
When you try to send a PM without a title, it errors. I had to adblock the sender div temporarily to fill out the title, then hit cancel on the adblock to bring it all back.