The height of rudeness? Or exceptions to every rule?

Do you think its arrogant to correct people?

I can't help it sometimes personally. Jamie Durie* was caught out recently, correcting Prince Phillip at the Chelsea Flower Show. Even thought he was most polite, the Prince took offence at his quiet correction

"He said to me, `I do like your tree fern'. I said, `Actually it's not a tree fern, it's a member of the cycad family. It's a Macrozamia moorei','' Durie revealed last night.

"And with that, he walked off. As he was walking away he said, `I didn't come here to get a lesson' under his breath. I didn't hear him say it, one of the boys heard him.''

In this instance there are several forces at play. Being from a monarchy we have a natural deference to Royalty, and it could be said that Durie shouldn't have corrected a member of the Royal Family (well he only married in anyway!)
He's also older than Jamie, so you've got a dose of "respect your elders" thrown in also.
But what about "the old bugger doesn't know what he's on about!" and "but this is my JOB"? (he did win the Show after all) I mean, Jamie Durie didn't go to University to study Horticulture only to let Macrozamia-ignorance waft by unchallenged.

I've learned to let mispronunciations pass me by. While this in itself makes me sound like a complete prat, sometimes I could hear people mispronounce words five or six times a day. It gets tiring trying to tactfully point out the correct way of saying something. *giant wank*

But when people use words completely out of context, or go off half cocked, I usually feel compelled to pull them up on it. Am I being a know-it-all? Or am I saving them future embarrassment by not correcting them, and allowing them to continue to think they're saying/using it correctly?

*Hehehe initially I wrote Drury, like Drury Lane, and the Muffin Man. Hahaha