Note, crash is resolved, but normal calculation logic looks like it may not be correct still. (though it doesn't result in visible errors, just extra calculations).
Win 7 64 bit
Broken: 2.76b official and build c986e93
As the title says; If you add a Mask modifier to a mesh (with a vertex group assigned), then switch to Sculpt mode...Blender will always crash for me. If you disable the modifier, switch to Sculpt mode and then re-enable the modifier...it'll crash once you start sculpting.
Exact steps for others to reproduce the error