They sent me the waitlist email 9 or 10 days after the acceptance email. I immediately emailed them to ask what was going on, and they responded to that email about 5 days later (I'll give them a break on that delay though because they were probably out of the office for the holidays). I really have no idea what I would have done if I'd withdrawn from other schools, there would almost certainly be lawyers involved though, I definitely wouldn't let an unorganized admissions staff member ruin my entire career
They apologized, but I wouldn't say they were extremely apologetic, and their "apology" email was actually pretty confusing.