"Don't swallow that gum! It takes seven years to pass through your digestive system!"
The Lie:
So there you are chewing gum and playing hopscotch while jumping rope on a roller coaster. You desperately try and keep that delicious resin-based goop in your mouth while enjoying your favorite activities, so one moment you're chewing on it, and then the next moment it feels as if a slug is climbing down your throat. It's then that your mother's words echo in your head, "Never swallow your gum, it'll take seven years before you crap it out." You suddenly picture years worth of gum accumulating in your stomach into a hardening clump that will, of course, kill you.
The Truth:
We would think you'd learn not to listen to your mother by this point, and that the only true thing she ever told you in your life was that you're adopted. The truth (as presented by Cracked.com and your friends at Hubba Bubba) is that you could swallow a whole thing of bubble tape and you'd get to see it come out the other end in a matter of days.
According to those killjoys over at Snopes, the reason your mom lied to you is because she was confused about the term "indigestible," which actually just means your stomach acids are unable to break down the components of gum. While this may sound unpleasant, note that no one ever accuses corn of ruining your digestive system, and it's just as hard to digest.
So the only thing this means is that your gum will appear intact upon exiting your body, which leaves it open to any number of workplace pranking opportunities, Cracked is not responsible for any injuries or firings resulting from that idea, but will gladly own up to any resulting hilarity.