Ice storm last week?? Pull back the cap at shoulder of windshield wiper arms.... 13mm nut secures arm onto *smooth* tapered fitting thats designed to let go when blades ice up. I believe this fail-safe feature first became available with titanium wiper arms of 107's and 116's.

Back in fintail days, splined hub was used and owners turning on iced up wipers burnt out their motors if they left em on.