System Information
Windows 7
Intel Core2 Extreme 3GHz
8GB RAM
NVIDIA GeForce 8800 GTS 512

Blender Version
Broken: 2.70

Short description of error
Bsurfaces addon crash bug:
'Add surface' crashes Blender under specific workflow. Caused by modifier data existing on a mesh (even if applied), before duplicating edge loops, separating and converting edges to curves, setting to bezier and applying 'add surface'.

Exact steps for others to reproduce the error
Enable the Bsurfaces addon.
Create a cube, subdivide a few times, remove half, add a mirror modifier, and a subdivision surface modifier.
Select three edge loops, duplicated them (shift D), then separated them (P).
Hide, delete or move the original cube to another layer.
Convert the object with edges to curves
Set the curves to bezier using the Tool Shelf, curve tools
Add a plane, shift select it and then apply bsurfaces 'add surface'.
This crashed Blender every time.

I tried a few things including applying the modifiers before separating the edges but it always crashed. The only way it would work was if there was no modifier on the subdivided cube before I separated the edges. Then it would work as desired. I didn't test those two modifiers individually, or any other modifiers.

Plus if I saved the file just before I crashed it and apply 'add surface' again I start getting error message after error message. Please try this to view errors.