It is not the 1st time I encounter this bug. Sometime vertex just could not be moved.

If I remember it right, one of those weird vertex is even a newly created one.I wonder if it is caused by some incorrect values on the vertex. e.g. vertex data object is reused from recycling and didn't get full reset on all the values. I did a little soft selection right before this happen. Not sure if this is related.
Guess what, I did a little more testing. It seems like the problem is related to links with nearby vertices. As seen from the 2nd video, a vertex moved to that particular spot linked with those lower left vertices would have position locked. However, what happened to those bad vertices at the corner is still a mystery.
