Here's a safe scenario: I use an L6-30P to 5-15R socket cord. It's a 25' or so extension caple that I used to just plug my L6-30P equipped Nissan Gen 1 EVSE into. It was modified to do 240V at 20A, hence L6-30 which is good to 24A. I replaced the L6-30R with the 5-15R.
Never had an issue...