My wife and I both enjoy teasing / lightly pranking each other. We had to institute a "safe word" to indicate when we're being serious and not setting up a joke.
(The instigating event for that was right before I introduced her to my grandparents and I really needed to convince her that grandpa was hard of hearing but too proud to wear hearing aids, so she'd have to speak loudly and into his left ear, not his right.)
In 10 years of marriage, we've never once violated the trust of the safe word. If one of uses it, the other drops any skepticism and takes it as gospel truth.