What lie did Tas catch Flint in?

Tas caught Flint in a lie regarding the reason why he had clutched his left shoulder. Flint had explained that the lance, thrown back by the force of collision had rammed into his shoulder, bashing it up good. Upon reflection later, however, Tas realized that the lance had been on Flint's right side, so there was no way it could have smashed into his left shoulder.