In the same line as the pornographic images inserted by prankster animators in Disney’s The Rescuers, I consider this one too coincidental not to to be deliberate. During the end credit for the 3rd Harry Potter film, Harry Potter and the Prisoner of Azkaban, footsteps are drawn walking around on the Marauder’s Map. At one point two sets of footprints appear to be having sex missionary style in the bottom right corner of the map.
Here is the relevant clip:
Appearance of the word ‘sex’ in Harry Potter Films
I’ve included only the most easily recognize subliminal here on this page, but others have been able to spot way more. I’m personally not able to see most of these, but have a go. (running time: 7 minutes)
Images source: Warner Bros. Harry Potter film series