SubSurface-Mirror Problem I am beginner, I am trying to made a car bumper and problem appears when I want to mirror my model (I modeled a right side half of bumper because is symmetric).

When I use only mirror modifier all is fine but when I want add subsurface modifier, my model has some bugs and mistakes.
The problem you are facing could be that the middle of your bumper model, where the mirroring starts, isn't completely flat along the x-axis. You can fix this by selecting the vertices, scaling and then pressing X and 0. 

After flattening the vertices, go to the properties panel by hitting N and put the vertice group to 0 on the x-axis.

Here are some pictures:


The situation


Select these vertices


Scale to 0 on the x axis


X co-ordinate to 0