One of the most common issues among both hobbyist and new professional photographers is the lifespan and "safe retirement" of memory cards. When is a good time to retire a memory card, or relegate it to "secondary usage only"? Well, any number of things can start happening to a memory card, from images going corrupt on the card noticeably often, to weirder things such as in our case today, where a professional wedding photographer noticed that their 16 GB memory card briefly appeared to only be 8 GB in capacity for no apparent reason. Sound like a harmless glitch? Think again! Even this subtle "weirdness" can be an indicator for impending total failure and data loss.
The very first thing that comes to my mind as an expert in this area is, ...where did you buy the memory card in question, and what "class" / speed and brand is it, plus of course how has it been treated overall?
In my opinion, there is a good chance it is time to at least relegate such a memory card to secondary duty.
HOWEVER, this "wrong size" issue is a common telltale sign of faux Sandisk cards from Ebay / Amazon third-party sellers, in which case you are at HIGH RISK of losing either half the card, or the ENTIRE card at any time, without further warning. And often times in this particular case there is no chance of data recovery, compared to how easy it can be to run a 100% successful recovery on a "legit" professional-grade memory card.
I know this sounds terrible and maybe even a little bit paranoid / overreacting, however considering the cost of memory cards these days and considering that wedding photographers in particular are paid professionals who are capturing once-in-a-lifetime images, I see very little reason NOT to replace such a card immediately.
I honestly just buy entirely new complete sets of memory cards every few years, card wallet and all. (After a few years of heavy use, the Think Tank Pixel Pocket Rocket that I love so much can admittedly start holding CF and SD cards a little loosely, so that's why I buy a new card wallet too. Plus I like to keep my old memory cards around, for emergency use and personal high-volume shooting.)
Of course I shoot every new memory card to 100% full a few times on non-essential casual work, (I shoot a lot of timelapses, which is a convenient hobby) ...just to test out the card. Or you Nikon users can just set your cameras to TIF mode, and fill a card in just 100-200 shots! Yes, I have had to send back a few cards over the years but that has only been when I tested out low-budget cards. The pro-grade, name-brand memory cards, purchased through an authorized reputable dealer such as B&H Photo Video, have NEVER let me down.
Anyways my point is, the safest thing to do is to start fresh every few hundred thousand images. I now have three separate card wallets, and in a pinch if I have to shoot 5-6 days of weddings back to back I might start using the older cards to "mop up" dance floor reception shooting at the end of the night, while using the more reliable cards for shooting earlier in the day. Again keep in mind, when I say "more reliable" I'm not even referring to cards that have given me issues in the past, I'm just talking about replacing perfectly functional cards that are simply 2-3 years old. Personally, any card that gives me a serious error such as this, gets immediately taken off professional duty, and goes in the bin for "random timelapse footage" and quick around-the-house shooting only...
So, it doesn't have to be that OCD / complicated. All I'm saying is, if this is your profession; be ready to invest in its upkeep / wear-and-tear!