beg your pardon, but

I beg your pardon, but (something)

I believe you are mistaken or incorrect; I beg to differ; I don't agree with you on that. I beg your pardon, but I believe you'll find that our school is actually one of the best in the state.
See also: beg, but

(I) beg your pardon, but...

 and Begging your pardon, but...
Please excuse me, but. (A very polite and formal way of interrupting, bringing something to someone's attention, or asking a question of a stranger.) Rachel: Beg your pardon, but I think your right front tire is a little low. Henry: Well, I guess it is. Thank you. John: Begging your pardon, ma'am, but weren't we on the same cruise ship in Alaska last July? Rachel: Couldn't have been me.
See also: beg, but