I've put frozen ground beef or frozen foods like manicotti in a room-temperature slow cooker. They cooked just fine. I don't take chances with ground beef any more (at the time I was fresh out of college, but not too smart in the kitchen), but frozen manicotti & sauce in the slow cooker is a good thing if you have to spend an afternoon at kid's ball games & you want dinner ready when you come home.

Hmm...I've put in frozen food for years, always before it's turned on. Chicken is what I do in it the most.
What's the problem with ground beef? Does it allow the bacteria to breed in the lower cooking temp.?

I think with all the e. coli & ground beef concerns these days, slow cooker mfrs., food scientists, & the beef industry prefer to err on the side of caution and advise people to "properly thaw" the ground beef first. Ditto the poultry with regard to salmonella.

I think today's bugs are stronger than Grandma's.

I was listening to a report on NPR several years ago about an e. coli outbreak at the time. The report said that today's e. coli is about as genetically similar to earlier e. coli as a human being is to a flea. Much more potent.