Win 10, GTX 560 Ti
Broken: (2.78 9d70344)
Worked: (2.77 abf6f08)
Vertex Connect does not work when adding extra vertex on edge is needed.
Vertex Connect bug occur after usually short time of modeling (randomly). When Vertex Connect stops working I'm not able to connect vertices when edges are between and it is needed to create new vertices.
Deselect vertices and exit Edit Mode can sometimes help to work again only for few next attempts.