Graphics cards are designed so that 2 adjacent triangles which share identical vertex positions will have no gaps or overlaps along the edge between them. But that only works out exactly right when the start and end of the edge is identical for both triangles. In the case of a T junction, You have 1 long edge trying to fit against 2 small edges. There's enough mathematical error in there to cause the long edge and the 2 shorter edges to not fit together perfectly, leaving gaps.
Actually I had red dots showing around edges in Thief 2 when my first geforce died.