I Know this is silly but this might be an interesting topic. Earlier today I was on Facebook, and my cousin posted a video on the topic of the Disney Animated Features with cameos of characters (or objects) from another animated feature! But the down side is that the Disney fans have theories that they are connected to each other (oh and Chris Buck who had worked on "Frozen" and "Tarzan" is also a Disney fan also has made fan theories.)

Connections aside do these crossovers apply to other cartoon studios?

I can think of some examples from the Fleischer Studios

I'm having trouble posting the two screencaps

Gabby from Gulliver's Travels

Superman: Terror On The Midway (the scene were the giant mad gorilla escapes, two clowns stare with shock, one of the clowns bare a striking resemblance to Gabby from Gulliver's Travels! (although slightly modified to fit with the rest of the design of the DC characters.)

Some Looney Tunes examples:

Colonel Rimfire reciting [tweety]'s catchphrase in [coolcat]'s debut cartoon

[bugs2] made cameos in several cartoons he didn't star in

[porky] 's cameo on a poster in Hare-um Scare-um

[lepew]'s cameo at the end of Dog Pounded

There weren't too many classic WB cartoons that featured a lot of the LT cast together, but one classic short that featured a variety of different character had to be The Scarlet Pumpernickel

[foghorn]'s cameo at the end of False Hare

Yogi Bear and Boo Boo made a cameo in The Flintstones episode "Swedish Visitors". Yogi was also mentioned several times throughout the series.

Even though not golden-age, Pixar does this sort of thing ALL the time. You can find the Pizza Planet truck from Toy Story in nearly every Pixar movie.