I have been seeing double for most of this week. At present nobody knows what is wrong with me. All I know is that I wore my new glasses all weekend, switched back to my contacts on Monday, and have had increasingly worse double vision since that time. Apparently I need to find a strabismus specialist or something.
UPDATE: This sort of thing is what I have been seeing. I can see normally if I close my bad eye. (Of course, this is the opposite of what you want to do, as it just weakens the already-weak muscle.) Double vision is not new for me, but in the past I have only seen this when I deliberately unfocus my eyes and I could re-focus them easily. Now I am having a lot of trouble focusing at all.
* Rejected post title: It's not a tumor!