Frozen
At Elsa’s coronation, two recognizable guests can be seen entering the palace grounds. Rapunzel and Flynn are spotted, and Rapunzel has her short hair as seen at the end of Tangled. There have been many instances of overlap within these two films, and it's likely that they exist in the same universe.
Frozen
The intro to the song “Love is an Open Door” in Frozen isn't actually an original idea. When the two sing “We finish each other’s…sandwiches!” it's actually a bit that first appeared on Arrested Development years earlier.