A WebmasterWorld thread shows an issue where an advertiser saw his average position drop from position five to position 195.6. adCenterEU, an official Microsoft representative explained that this is indeed a bug in the system and Microsoft is working on a fix.
There is a known issue with some advertisers where the average reported position is higher than expected.
The adCenter team are aware and hope to release a fix as soon as possible to recitfy the behaviour.
So if you see your ad position drop like that, do not fret - it is a known issue.
Forum discussion at WebmasterWorld.