System Information
Debian 8, nVidia GTX970, intel 4770K

Blender Version
2.74

Short description of error
Display precision of 3D cursor in View Properties (N) window is just 4 digits, which is lower than the tolerance of mirror editing.
Also the display precision of the 3D cursor is 1 digit less than the display precision of vertices in the N panel.

Exact steps for others to reproduce the error
In a mirror editing session I accidentially moved one vertex with mirror editing off. To fix this, I snapped the 3D cursor to the vertex, flipped the sign of the 3D cursor X coordinate in the view properties (N) panel and then snapped the mirrored vertex to the 3D corsor.

But this didn't work. Mirror editing still thinks that the two vertices are different. The reason is that the vertex coordinate is shown as -0.10377, while the 3D corsor coordinate is shown as -0.1038. When I invert the 3D cursor, it will be 0.1038 rather than 0.10377. So I guess the display resolution of the 3D cursor is below the tolerance limit of mirror editing.

I guess increasing the display precision of the 3D cursor by 1 digit (same as vertex display) shoudl help.

Best regards,

Michael