I don't know if anyone else has had this experience, honestly. Or, more accurately, I'm not sure if anyone else has been conscious of having this experience.
You've made a character, and you like them. You use them in a scene or two, but Real Life has its talons in you, so you shelve the character and those scenes, thinking you'll come back to them later. But you don't. Because things happen, or inspiration strikes from another angle, or the holidays come up, or or or.... And all of a sudden, you realize that you've forgotten most of what you developed for that one character and you might not even remember their name.
Or is it?
Okay, so here's the deal. When a character is "alive" to me, they're like a voice in my head. (Hang on, stay with me. I'm not crazy.) So when I'm working on a project, those characters live in my head and what I write is basically what they tell me, polished up a little bit.
But if a character is left behind, if they're put on the shelf or dropped or killed off or otherwise I simply don't have time to interact with them for a significant period (say, 2-6 months) then that character "fades." They're not "alive" in my head anymore. I don't hear their voices, and their personalities (if they had any to begin with - ouch, hard truth) begins to disappear. I can't hold onto an infinite number of characters at a time, so the ones that don't get a lot of attention get forgotten.
This happened recently with a character I named Christine, or Chrissy for short. I wrote down her features (basically what she looks like, her preferences for friends and crushes, her family and career, etc.) so I would be able to come back to her later, but I left her alone for so long that her personality more or less disappeared. I have it written down that she's "curious and friendly," but that's it.
So... is this necessarily a bad thing?
I don't think so. After all, if I'm not actually using the character, it seems unfair to keep them around just... doing nothing. On the other hand, the space in my brainpan is limited, and discarding nonessential information is part of the process that keeps me sane from day to day. (Just imagine if you had to sort through every stupid piece of trivia you'd ever picked up every time you wanted to remember how many feet there are in a mile. That'd be enough to drive me insane, for sure.)
It can be very disappointing, though, when you don't have any notes about how the character is supposed to act, so you can't recreate them properly next time you put them down on paper.