It doesn't bother me as such, but I lose a little respect for everyone who uses "your" for "you're" and so on.
What makes me despair a little is shop signs advertising things like "CD's" or worse "computer's" (one could make the argument for the ' filling in for missing letters).
If you've hired someone to write signs for you, and you don't have the basic English required to correct their mistakes, you probably shouldn't be running a shop!
The problem is not so much on bulletin boards, but more that this is real life. We get letters from official bodies with bad grammar - to point it out is pedantic, but would you trust a bank with your money if they can't even write a letter correctly?