Ever seen Article 14 of the Panama-Costa Rica 1973 preferential trade agreement? It’s an anti-MFN clause:
The Contracting Parties agree not to grant, through a most favored nation clause, the concessions they grant to each other through this Treaty now or in the future to third countries from outside Central America with whom they may sign trade treaties or conventions.
The two countries signed a PTA in 2007, which probably didn’t include such nonsense, though I can’t find the text.
[Hat tip: Lorand Bartels]