XP used to do it. I found it really hard for ages to differentiate between deliberate, abusive gaslighting behaviour and the other kinds - the trying to get out of something/"selective" memory kind, and the genuine forgetfulness. I can see the difference now I'm in a normal relationship and DP does the forgetful one and occasionally the selective memory thing
Basically in order of harmlessness:
Genuine forgetting - when you both think one of you said one thing and the other thinks another and in the end you both laugh and say "Oh who cares"
Selective memory - this is things like when you tell them about something they don't really want to go to so they helpfully claim that you never told them and they didn't want to go to. Or when they agree non-committally to do something and then deny they were ever asked. This is annoying and slightly manipulative but at a low level it's not really a big deal - if it's all the time though or on big things they KNOW will upset you or inconvenience you then it probably is bordering on malicious/abusive.
Then probably what I would term "true" gaslighting - this is the kind which sounds so insane it makes you doubt yourself and you know nobody would believe you if you questioned it anyway. One example XP did was that one day he'd put the oven on to heat up, I got to it to put whatever it was in and said "Tch, you only put it on 140, we'll have to wait a bit longer for it to heat up now" and turned it up to 200. He said "No I didn't, I put it on 200" and continued to argue this for about the next ten minutes while we both got more and more irate because I knew I had just turned it up and he just SAW me do it and why the fuck would I have needed to do that if he had got it right? This argument culminated in me asking why he thought I would lie about it and his answer was "To make me look stupid".
There was NOBODY THERE.
Another example I remember on a thread was where a MNer had two pasties in the fridge, one chicken, one beef. Her H ate one of them and then swapped the packets. So he left the beef one in the chicken packaging. And then he watched her while she ate it, almost daring her to comment on it. Or something like that - it was odd. Really weird. Just totally NOT something a normal person would even dream up, even as a joke or something.
Reaction is a good gauge as well - if you can both laugh it off and say it doesn't matter it probably is more likely to be innocent than if they push the issue and get annoyed at you or say things like "I can't trust you to remember anything" or "You must be going mad" etc.