As a CMU alum, I agree that it’s quite embarrassing. The odd thing is that it keeps happening. One would expect that by now, every admissions office would have super locked-down, triple checked procedures in place for sending out acceptance notifications… something akin to Microsoft or Apple pushing out a major software update, vs. a staff member pushing the wrong button.
Of course, these screwups made the news back in the days of mailed acceptances, too. Every now and then the wrong group got fat envelopes, and it took longer to sort out the ensuing mess.