I finally, after several months, was able to get back in and play in the darkroom. Everything seemed to be going nicely, but when I looked at one print in the light, I noticed a strange looking blotch on it. (See attachment)

My first thought was back to the negative. Perhaps I screwed up its developement or something. After re-inspection, I didn't see any problems with it. The blotch doesn't appear on any other prints from that session and, because I thought I got this one right, I didn't make any additional prints of this.

Any ideas? I'm still scratching my head...