Texas did something like this to me. I decided on Northwestern over the weekend, and I withdrew from other schools I was admitted to via e-mail to Michigan, GULC, Vandy, GW, and Texas on Monday. Most schools sent me nice, brief notices that they got my withdrawl.
Texas, two days after I withdrew, however, sent me an e-mail (from the same address I sent my withdrawl to) saying I had missed the deadline to turn in my seat deposit (either April 15th or three weeks after acceptance) and rescinded my admission. So I sent them a quick e-mail back saying that I had withdrawn my acceptance two days earlier, but thanks for the notice! It upset me a little because they had been so nice throughout the rest of the admissions process. Either they are really disorganized or this is a shady way for them to improve their selectivity (I know they must be hoping to break into the magic *top 15* that Leiter talks about, hehe).