When people do this it's important to say something to them about it to send the message that what they did is not acceptable, otherwise it sends the message that "corrective misgendering" is acceptable or accepted when it isn't.
I am personally not in a postilion to correct people right now as I am still using my dead name with some people... in the future though I would LOVE to just say "no need to apologize, you got it right the first time." Keeping in mind that for those that are working retail, it is probably best to politely correct whenever possible.
Now just out right aggressive shit like when I heard some guy behind my back say something along the lines of...
(was going to leave a very upsetting example here but decided to spare you the example, but let's just say that while I am boymode-ing often it is obvious to many that I am either trans or queer based on my appearance)
...as I was still working I just turned around and while my voice and words were nothing but polite... I am sure my expression did the trick because the dude shut up fast.
(I hate my body (my height being one thing I wish I could change), but in situations like that, an upset expression from a 6'2" person is somewhat more affective)