We report an exceptional case of a tibial stress fracture combined with a large avulsion from the posterior facet of the tibia at the level where the tibialis posterior and soleus muscles insert. This type of injury has not been reported previously. A young healthy male runner experienced a sudden "snap" in the right lower leg 25 km into a marathon and had to leave the race due to increasing pain. The fracture was immediately apparent on plain radiographs. The injury was treated conservatively with 9 weeks in an ROM knee cast and no weight bearing on the affected leg, followed by full recovery. We propose that prolonged muscle tension at the site of tendinous attachment to the bone can cause the development of a stress fracture type of avulsion.