Now, the 'just check the thread' advice falls apart in a few situations. Some batteries use a proprietary connection—like a magnetic adapter or a different thread pitch. You'll see this in some sleek, small pens. If the gift recipient has one of those, a standard 510 cartridge won't work without an adapter.

Also, not all 510 threads are created equal. The threading can be slightly different in depth or pitch—though that's rare. More common is the cartridge's base being slightly larger or smaller, which affects how tightly it screws. A loose fit can lead to poor contact.

And then there's the 'fit' beyond the thread. For a gift, you want it to look right. A 2ml cartridge on a tiny pen looks ridiculous. A small cartridge on a big battery feels off balance. So 'compatible' doesn't mean 'looks good.'