I know what you're talking about and I hate those little undeveloped seeds! I've only come across it a few times and the last time I was sick of putting up with it after paying top dollar for an eighth. Fortunately I picked it up at one of my favorite shops and they let me bring it back in and exchange it with no problems!

I suspect, at least in that instance, that the buds got pollinated by intersex male flowers very shortly before harvest so the seeds didn't have a chance to fully develop. I think in the shop they said it was from some late hermies caused by light issues. It made the smoke really unpleasant for me. There were basically pinhead sized hard things scattered all throughout the bud and they tainted the taste:rasta: