Minor gunk under the cap

[BGFreak99]

Hey, all,

I recently bottled an Oktoberfest, and I've found a couple (out of the ones I've opened so far) that have had a thin ring of grayish stuff around the rim. The beer itself tasted fine - no different from the others that have not had the gunk. I'm just curious as to what it is. I haven't encountered it in any other beer I've bottled (the Oktoberfest was my 6th brew). Anyone know?

Thanks,
Shane