i find the words to describe the mentally handicapped offensive to be honest, most of the others are just shortened versions. I dont think the words spastic/mongol are offensive, just old terms that are no longer in use or shouldnt be, but spag and mong/mongy are dreadful in my opinion, the latter forms of these words, mongol and spastic were used by kids to call people, certainly when we were kids.
I think there will always be something that offends someone else though and we mustnt get too upset by ignorance.
when i was working with the mentally handicapped in a hospital, i used to read through the old notes from the 30's 40's etc.... and it made for shocking reading, the terms that were used to describe them were horrendous! Things have changed for the better but we need to be realistic with what we can and cant say. A lot of it is way too OTT with politically correctness.