While talking with my wife recently, the friendship level pop-up in the dialogue box shows we have level "13/12":
This seems peculiar, since only 12 hearts out of a maximum of 12 show in the social tab:
Furthermore, on the wiki (here and here), the maximum friendship level is stated to be 12 for a spouse. From the second link (emphasis mine):
Once a villager has been married, they will move into the farmhouse. They will have a maximum of 12 hearts instead of 10 [...]
How is this possible that the friendship level is above 12, which appears to be the maximum?