cam chain tensioner gasket leak???

[MrJoel]

How common is that gasket to start leaking? I thought that the oil pressure sensor o-ring was leaking due to the amount of oil around that area, but when I took the headers off I traced it back up the front of the bike starting under the tensioner adjustment. It doesn't appear that it is coming from the valve cover and there are no signs of the head gasket leaking, just seems odd that that gasket would fail