I know that scenarios contrary to fact should be in the subjunctive mood, but ones that we are uncertain of can use past tense. For example, "If I were you..." vs "I don't know if I was right." But what about the sentence "I didn't think it __ possible," assuming whatever is talked about did happen? It's not contrary to fact, but the speaker thought it was contrary to fact at the time. Both "was" and "were" sound right to me.