Mmmh there is rather an easy way to get that bug due to float precision is when the vertice positions are the result of some maths. Then two vertices which should be mathematically at the same position will end up not being exactly at the same position in practice due to float precision. Only ways to have two vertices at the exact same position is either hardcoding it, assigning the position of one to the other or using index buffers. An easy thing to miss.