Possibly the oddest cartoon crossover ever is this Warner Bros/Filmation mash-up from 1972. It was made as part of ABC's Saturday Superstar Movie, which featured one-hour specials from various animation studios (Hanna-Barbera, Rankin-Bass, etc.).
The Groovie Goolies was a series produced by Filmation, originally as a segment on Sabrina the Teenage Witch (1970). They were popular enough to get its own spinoff a year later. And then, finally, they appeared in a crossover movie with the Looney Tunes characters.
Like everything Filmation has done, this is a poor cartoon. The animation is barely-there, the stock music is grating, and the laugh-tracks are cumbersome. Mel Blanc voiced all the Looney Tunes gang, but they sped up Daffy's voice too much, to the point that he sounds like Woody Woodpecker!
It's unlikely that this special will ever be re-released due to the rights issues. Classic Media owns the rights to most of the Filmation properties, including the Goolies, while the Warner Bros., of course, has the Looney Tunes. After seeing this, though, I don't think anyone's itching to buy a DVD of it except for die-hard completests.