I have Main office and branch linked with 2 TMG 2010 (+ all SP's and all rollups) over PPTP site-to site.
Sometimes some clients can't access to main office sourses with 0xc0040014 FWX_E_FEW_SPOOFING_PACKET_DROPPED on branch's TMG; internet sill works.
Everytime diffrent client, but no more then 1 at once.
There are 2 providers on branch office with ISP.