A friend asked about the correct plural form of the vernacular "pjs" for pyjamas. Pyjamas is, of course, a permanently plural word - no one says he or she has just one pj. I answered that since "pjs" is not the combination of two words, instead being closer in nature to an acronym, an apostrophe indicates possession rather than plural. An example: The Waldo County YMCA's gym is quite expansive, while other YMCAs seem to have smaller gyms.
I now discover there's a hot debate associated with this. English is a fluid language, changing significantly with every generation. Apparently, the opposing side of the "pjs" debate ("pj's") argues that the apostrophe makes the term more readable. As my brain does a mental jerk every time I run across this error, it has the opposite effect on me. However, I am willing to accept this evolution as a stylistic preference. If you tell me, prior to editing your work, that this is the way that best suits you, I'll leave it alone and keep my mental spasms to myself. :)