It looks like it is getting ready to flower. Check out the fat yellow thingeys. I would try to pick it a little earlier. I have let it flower and the flowers are edible. (After I harvested the main bunch, that is).

No, sorry, not the blossom, but further down. See how the buds are distinctly yellow and have even fallen off on some of the florets lower on the stems. Or maybe they failed to develop?

did they get a little black rot perhaps?

The only thing I can think of is that it might have gotten frost damaged. Although I don't know how only the side pieces would have been damaged by frost and not the main head.

Rodney

Maybe if you had cut those heads a week ago, they would be perfect. Looks like the same old-age yellowing you see on all big brassicas, and your beads are on the verge of opening. Better than armyworms.

Thanks for all the ideas! But none of them quite seem to fit. I'm not really worried, I just want to understand what I'm seeing. I also grew many perfect heads. I went and looked for further pictures on all the suggested possibilities, and they aren't it as far as I can tell. It doesn't feel like a rot, no squishiness or odor, it's not frost damage since it first was noticed before any frost, and it also was showing up before these heads reached maturity, so I don't think it is age. I just got around to taking the picture this week, partly because I wanted to see how it grew out.

Another piece of the puzzle is that it is only happening on one cultivar (Packman). Could it be some sort of slight nutritional deficiency? Maybe an environmental stress? It was pretty hot (85-90ð) and dry right up to the 10th or so, then rapidly dropped about 40ð and drenched us with a week of rain, then once the rain stopped we had our first frost a few days ago.

Anyway like I said earlier, I'm not losing sleep over it, I just wonder what those plants are telling me. Cheers!

This is a condition known as brown bead. Multiple factors can cause it, but often associated with stunted growth. Some varieties are much more susceptible than others. http://growingsmallfarms.ces.ncsu.edu/growingsmallfarms-broccoliproblem06/

Here is a link that might be useful: Brown Bead

Yes! That looks just like it, thank you! Since stunted growth was not a problem, and high temperatures were listed as another associated factor, I'll figure it was that heat we got just before the rain. I get great satisfaction knowing what that is.