The L.A. pitcher was working on his off-speed stuff in an at-home, quarantine throwing sesh this week ... when he let one get away from him -- and threw the cowhide straight through a window!!!

Yeah, JUUUUUUUUUUUUSTTTT a bit outside!!!

You can see in the footage of the aftermath (filmed by his wife) ... the damage Kelly's changeup did was pretty significant.

There was glass everywhere and a big ol' hole through the window ... and JK sure seemed to be in disbelief over it all.

Don't worry, Joe's got enough funds to pay for a new piece of glass ... dude signed a 3-year, $25 MILLION deal with the Dodgers just before last season.

Also, it coulda been much worse ... at least it wasn't fastball practice -- the guy throws his heater 100 MPH!!!