In philosophy, the Principle of Charity is a technique in which you evaluate your opponent’s position as if it made the most amount of sense possible given the wording of the argument. That is, if you could interpret your opponent's argument in multiple ways, you would go for the most reasonable version. This is a good idea for several reasons. It counteracts the illusion of transparency and correspondence bias, it makes you look gracious, if your opponent really does believe a bad version of the argument sometimes he’ll say so, and, most importantly, it helps you focus on getting to the truth, rather than just trying to win a debate.
Recently I was in a discussion online, and someone argued against a position I'd taken. Rather than evaluating his argument, I looked back at the posts I'd made. I realized that my previous posts would be just as coherent if I'd written them while believing a position that was slightly different from my real one, so I replied to my opponent as if I had always believed the new position. There was no textual evidence that showed that I hadn't. In essence, I got to accuse my opponent of using a strawman regardless of whether or not he actually was. It wasn't until much later that I realized I'd applied the Principle of Charity to myself.
Now, this is bad for basically every reason it's good to apply it to other people. You get undeserved status points for being good at arguing. You exploit the non-existence of transparency. It helps you win a debate rather than trying to maintain consistent and true beliefs. And maybe worst of all, if you're good enough at getting away with it, no one knows you're doing it but you... and sometimes not even you.
Like most bad argument techniques, I wasn't aware I was doing this at a conscious level. I've probably been doing it for a long time but just didn't recognize it. I'd heard about not giving yourself too much credit, and not just trying to "win" arguments, but I had no idea I was doing both of those in this particular way. I think it's likely that this habit started from realizing that posting your opinion doesn't give people a temporary flash of insight and the ability to look into your soul and see exactly what you mean—all they have to go by is the words, and (what you hope are) connotations similar to your own. Once you've internalized this truth, be very careful not to abuse it and take advantage of the fact that people don't know that you don't always believe the best form of the argument.
It's also unfair to your opponent to make them think they've misunderstood your position when they haven't. If this happens enough, they could recalibrate their argument decoding techniques, when really they were accurate to start with, and you'll have made both of you that much worse at looking for the intended version of arguments.
Ideally, this would be frequently noticed, since you are in effect lying about a large construct of beliefs, and there's probably some inconsistency between the new version and your past positions on the subject. Unfortunately though, most people aren't going to go back and check nearly as many of your past posts as you just did. If you suspect someone's doing this to you, and you're reasonably confident you don't just think so because of correspondence bias, read through their older posts (try not to go back too far though, in case they've just silently changed their mind). If that fails, it's risky, but you can try to call them on it by asking about their true rejection.
How do you prevent yourself from doing this? If someone challenges your argument, don't look for ways by which you can (retroactively) have been right all along. Say "Hm, I didn't think of that", to both yourself and your opponent, and then suggest the new version of your argument as a new version. You'll be more transparent to both yourself and your opponent, which is vital for actually gaining something out of any debate.
tl;dr: If someone doesn't apply the Principle of Charity to you, and they're right, don't apply it to yourself—realize that you might just have been wrong.