shift select stops working?

I've ran into this problem before and wasn't able to fix it, I ended up just exporting/importing everything into a new scene which is obviously less than ideal.

I'm working on a file, and I must have accidentally hit some kind of hotkey or clicked a box where I cannot shift select multiple objects/faces/edges/verts. I've scrolled through the options and preferences trying to check/uncheck boxes that looks like it might do the trick but I haven't been able to figure out what the secret is just yet.

Anyone know how I could fix the issue?

I'm using smardac's custom interface if that matters.

No idea what this could have done, but I added a new cube object to the scene and shift select worked again.

Can you reproduce it?