"Lee, interesting that you had the same experience in the same condititions, were your negs printable?"
Yes, they were all printable although exposure times were longer than I'm used to. It's still a mystery to me why it happened. I haven't had the experience again.
Even so, I think 25 minutes at 1:50 is somewhat longer than optimum for HP5+. I think if you were to do a test where you develope a whole roll of the same subject and exposure, removing a few frames from the developer at, say, 3 minute intervals after 15 minutes, you might see that the contrast had reached its maximum at around 15 and the overall level increased after that. I'm thinking of a set of curves Phil Davis published some years ago in Photo Techniques, maybe while it was still Darkroom and Creative Camera Techniques I can't find my copy right now. The contrast vs development time for Rodinal flattened out fairly soon. After that point, any change in density would give more fog.
Thanks Gainer, that sounds like a good article.