Personally, it’s nbd when people slip up - especially people who’ve known me for a very long time pre-transition. Oftentimes they correct themselves, and I usually feel worse that they feel bad about it. It’s pretty easy to tell when it’s intentional or not, and I reserve my ire for people who clearly mean disrespect.
Though, I should say, that’s now - early on in transition, it was certainly a bit harder to take. It reminded me of very fresh family abandonment and abuse over my identity. That’s not on the people who accidentally called me by the wrong pronoun, but it certainly could put me in a pretty bad place and I’m sure I wasn’t the friendliest in those moments. The more that trans folks are supported by their friends and family, the more secure they feel and the less likely they are to react strongly to being accidentally misgendered, imo.