When a company owns the rights to multiple franchises a few cheeky cameos are to be expected. Something for serious fans to watch out for or the foundation of greater expanded universe (the disaster that was AVP started out life as a brilliant little easter egg), its all a bit par for the course in an age where every piece of movie trivia ever is at our fingertips.
The odd cases (eight of which Ill have a look at today) are where the cameos make absolutely no sense. Maybe the company didnt own the rights to the character they were guest starring or these two worlds had no business colliding, but all of these are as ridiculous as they are brilliant.
Honourable Mention - Pixar
Unless its your first day on the Internet, it should come as no surprise to see Pixar appear on this list in some form. Since their very early short film days there was a crossover of designs (a mixture of saving time by reusing animation models and honouring their previous successes) and as times gone by its only got more prolific; beginning with the Pizza Planet Truck becoming a recurring joke and the chess player from short Geris Game popping up as a toy repairer in Toy Story 2, were now accustomed to upcoming characters making little cameos in the latest film.
Jessie and Nemo are in Boos room in Monsters, Inc., Dug scares Remy in Ratatouille, Meridas family appear as vehicles in a tapestry in Cars 2; you get the idea. However, theres such an intrinsic link between all these characters that it doesnt really deserve its own entry, but Im always up for giving such gleeful creativity a nod.