It came to my attention, maybe in this recent 2.71 built, that there is a big problem with the Mirror Modifier. Whenever I use it for modelling, everything is good until I need to start to touch the gap just at the middle of the model.
For instance, whenever I needed to move the vertices at the center, to create new geometry or just because some vertices that went too close to the center automatically closed the gap, I usually would turn the Clipping and Merging OFF to be able to move them away from the center along the X axis and turn things ON again. However, recently this is not working. No matter, even after I delete the Mirror Modifier itself, I can’t move any of the vertices around the X axis to make them pop away from it. What is the problem here? I knew I managed to fix this on an old model (a week ago!) but I can’t remember how I did it, so, if someone suffered from this bug and knows how to fix it, please, share!