Grandma's Marathon in Duluth, MN. I was attempting a BQ (3:20). The weather was good except for the 15-25mph headwinds. I stayed with the 3:20 pace group through the halfway point, then I started dropping back a little. Somewhere in mile 14 I took my first walk break to try to loosen up tight back and leg muscles. After that it just went to crap. My right calf kept cramping worse, and I kept walking more. By mile 19, I stopped, sat on the side of the road, and tried to massage it out for a few minutes. I kept trying to press on, but by mile 22 I knew it wasn't worth it to me to walk it in. I was afraid of doing long-term calf damage, and I didn't want another 4+ hour marathon. A DNF sucks, but it's much better than an injury.
After I got home on Sunday, it occurred to me that my right calf also cramped up really badly a few weeks ago (5/23/10) after a weekend of hard trail and PWP running. That Sunday, it was fine on the run, buy when I was swimming with fins later, it totally locked up. My Grandma's cramp problem was in the exact same spot. :-(