Long long ago I went to Denny's for dinner, for some reason, and we ordered "fried mixed vegetables". They brought us canned vegall driping in grease. They did take it back. These cooks at various places often are very poorly trained, or quickly trained on some menu items. It was probably a Tuesday and we wanted to eat dinner without any hassle or much expense.
Denny's used to have a Grand Slam Breakfast for $5 or something that I'd get on occasion. On the whole, it is in the running for worst chain IMHO.