I voted "sometimes."
For the most part, they don't matter or are easily fixed. On occasion, they're major gaffes. Sometimes, I can just say "I goofed- this is how "X" should be done, and we'll play it that way from now on." On the other hand, if I goof in such a way that it actually makes the game a bit more fun, I keep my error in as a retconned homebrew rule...still while letting the players know what's up. I wouldn't want them to try gaming at someone else's table thinking my error was the way the game was meant to be played.
The last campaign I ran, the main foes in the first story arc were a homebrew...and I screwed them up royally- I based their special attack mechanics & tactics on the wrong stat, making them far too easy to defeat at higher levels. Worse still, I didn't figure that out until AFTER the players had gone through the entire initial arc, somewhat underwhelmed by the non-epicness of the epic final battle.
I apologized to them, but didn't get to rectify my error with the subsequent story arc- the group broke up over RW non-game issues.
You're (presumably) human- you're going to make mistakes. The trick is to minimize their negative effect on the game.