Guinea pigs, like rabbits, can be prone to teeth and jaw problems. This is mainly due to the fact that their teeth grow constantly. Signs that indicate a teeth or jaw problem include a change in eating habits (i.e. eating less, choosing to eat soft vegetables, not eating hay), increased salivation and a change in the angle of the occlusal surface of the incisors (the surface where the incisors meet). If you notice any of these signs, you should consult a vet.
Normally, the incisors teeth will wear completely horizontally. If this starts to become a slope (as in Myrtle's case as seen on Facebook), it is a sign there is a problem with the way your guinea pig is eating. This problem could be mouth pain (caused by wounds or molar spurs) or jaw pain (most commonly caused by abscesses).