(as described here: http://forum.openx.org/index.php?showtopic=503427566
after upgrading from 2.6.4 to 2.8.1, we noticed a huge discrepancy for some ad types. where normally, the difference between requests and actual impressions were around 2-5% (in 2.6.4), 2.8.1 had up to 60% less impressions than requests - and than before with 2.6.4. so 2.8.1 still counted about the same amount of requests as the day before (on 2.6.4), but only about average half of the impressions.
as far as i could tell, all the affected banners are have flash based creatives, hosted by openx. all of them have delivery limitations set to particular urls, and some of them have a capping, others don't.
other campaigns, for example image-based ones, do not have this problem at all. their request/impression numbers match more or less.
as of course loosing half of the impressions was not acceptable, thus we went switched to 2.6.4/5, and the impressions got back to normal numbers instantly. while i doubt that it is our environement which causes this - again, in 2.6 everything works just fine - , i can be contacted for direct access to both our 2.6.5 and 2.8.1 installations if needed.