Today I learnt that you can have only up to 3000 photos on your 3DS's SD card. No matter how much space you have left on the card, no matter if they are 3d or 2d photos (a 3d photo is, technically, two files), if it can be accessed through the camera application as a photo, then it is a photo and there are only 3000 of those to be had.

Corollary to this is, if it can't be accessed by the camera app, then it doesn't count towards that number. So if you put the card in a computer and rename the files or move them around, your 3DS's software might not recognize them anymore.

According to this thread on the Citra forums, you can get around the limit by deleting the cache file found at [3DS SD card]/Private/00020400/phtcache.bin (Citra is an emulator, but the same file exists on the SD card of a physical 3DS console). Or at least, you can get out of it if you moved/deleted your photos and are still stuck at the limit somehow. So that's interesting to know.