There is a vlog series I quite enjoy Matt and Tom's park bench and one thing they have discussed a couple of times is that the London underground has a number of signs that are used for important information and warnings like "The Piccadilly line is not running today".
But sometimes, particularity around holidays like Easter, Christmas, and New Years they put up jokes on the signs like "We wanted to make a joke about Easter but couldn't think of one that was very bunny."
For someone who is fluent in English that might just be mildly annoying because you look up, see there is a notice on the LCD screen, worry that your train has been delayed, wait for it to scroll across before realising it's just a lame pun and then move on. For someone who is not fluent in English, it just adds to an already stressful situation.
The same should apply to UI design and security systems. I love a bit of humor in life, and some projects get this right. Python, for example, has a great mix of humor and in-jokes in their documentation to stop it from being too dry. But they don't put jokes into the important parts of the documentation or in error messages because the last thing some poor newbie who is debugging wants to read is some witty jokes that don't help them fix the problem.
It really shouldn't have to be said but:
When designing a system, don't put jokes in warnings or in functional bits of documentation.
