I have read about folks putting gas in a diesel tank, and/or diesel in a gas tank. Usually hear about it on forums from someone that heard about it from someone else that heard about it.....
I know people read stuff and immediately believe it, but based on my limited experience (someone that actually has a truck which uses DEF) I think it would be impossible to put a diesel nozzle, which even the small ones measure about 1 3/4" wide, into a DEF inlet which are about 7/8" wide. I mean come on, when that big thing won't go into that little thing, no one that is sober is going to just pull the trigger and start shooting diesel toward the DEF opening!