I knew about the part that juts into Waukesha County. Didn't know about the Washington County extension. Back in the last century (don't remember when, probably in the post WW2 annexation craze) there was a movement to annex a large corridor of land abutting, IIRC, Hampton Ave that would jut out a few miles into Waukesha County. I'm assuming incorporation efforts by Menomonee Falls put a damper on that.
I've always been a geek about stuff like that. For example, from Good Hope Rd south to Burleigh, and again for about a two mile stretch on the south side, there is no 59th street. It's 60th, then 58th. Never knew the reason for that.