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One thing to check for: has your product or service been featured or mentioned in some way in the media? (Perhaps check with Google Trends?)
I've seen examples of 100x (and more!) spikes in impressions in the past, when something is all of a sudden being talked about by a major TV personality/icon whose first name begins with the letter 'O' - or in similar situations. ;)
I called Google earlier. They said there was an issue. No idea if it's resolved yet.
Since seeing your second post, Soze, I've consulted two colleagues on the tech team, one a senior manager, and they have not heard of this as an issue. I wish I knew who you spoke with, as I'd like to check with them. :) If you have their name, perhaps you'd sticky me?
In any case, I'll keep my ear to the ground.
Anyone else seeing this sort of thing?