Dear Boston Red Sox Players, This shirt is incorrect.
It should be, "If you see this, you're in second."
From Grammar Girl:
Your, Y-O-U-R, is the possessive form of you. Does this belong to you? Is this your strangely worded ad campaign? Your: Y-O-U-R.
You're, Y-O-U-apostrophe-R-E, is a contraction of two words: you are. Remember that an apostrophe can stand in for missing letters, and in this case it stands in for the missing letter a. It doesn't save much space, but it does change the phrase from two syllables—you are—to one syllable—you're, so I guess it serves an honorable purpose. You're the best listeners ever! You're: Y-O-U-apostrophe-R-E.
Thank you for playing and please tip your waitress.
Love & Kisses, Stacey