I can think of other things as well, at least in some cases: here I'm thinking of baby forums that extensively use their own semi-secret acronyms and other coy phrases.
I haven't frequented those forums at all, but I've read about other things, such as odd choices for baby names, and sometimes the discussion drifts into talking about the language on baby forums. A search easily yields multiple complaints online that this style of writing makes understanding posts harder for beginners and other outsiders. I've speculated, and others have posted in discussions, that maybe part of using some secret language is precisely to create a sort of in-group. I sympathize a lot more, though, with the uninitiated parents who simply want to discuss a matter without having to learn a sort of second language, especially because it's simply unnecessary for plain discussion.
In fact, here's one example of using the same acronym in different ways. On those baby forums, "BM" can mean "breast milk." As you readers probably already know, "BM" is more often used for something rather different. If you're anything like me, you'll have great difficulty reading certain baby posts without bursting into laughter--and great difficulty understanding how anyone who knows the more common meaning of "BM" can use it to mean "breast milk" as well.