Have there ever been cross network crossovers that aren't caused by the same production company?

For example, Detective John Munch played by Richard Belzer started out on Homicide: Life on the Streets. He's gone on to crossover (https://en.wikipedia.org/wiki/John_Munch#Appearances_and_crossovers) to 9 other shows on 5 different networks, with varying production companies.

Flash and Supergirl had a crossover (https://en.wikipedia.org/wiki/Worlds_Finest) before Supergirl got moved to CW. Similarly Constantine got cancelled on NBC, and short time later his character (https://en.wikipedia.org/wiki/John_Constantine_(Arrowverse)) got introduced in Arrow on CW, and has now become a main character in Legends of Tomorrow.

Danny Pudi appeared in an episode of Cougar Town in character as Abed from Community, after telling a story on Community about being an extra on Cougar Town. Both had completely different production teams, networks, etc.

And then there's the Tommy Westphall Universe Hypothesis (https://en.wikipedia.org/wiki/Tommy_Westphall#The_Tommy_Westphall_Universe_Hypothesis).

Ally McBeal on FOX and The Practice on ABC had a crossover. The funny thing in Fort Wayne was they aired at the same time. This was before Indiana observed daylight savings time, the big three networks aired primetime from 7-10PM, but our FOX affiliate aired primetime from 8-10PM. So both shows aired simultaneously at 9PM. And this Tuesday on March 31, a character from Chicago P.D. will cross over to what is now the FBI season finale.

Buffy and Angel ended up on different networks (Buffy moved to UPN; Angel remained on The WB), but they still had some crossover episodes after the network change.