Sounds like it's a fairly common problem and it has been happening to me for a while (first time was 2 and a half years ago) and it seems to happen on more sensitive areas (inner thigh, inner chest near put, etc).
The previous areas were all on clean skin so they have since been tattooed over but I am worried that it might scar if it happens over a tattooed part of my body and I would be pretty upset...