I suspect typos in the keyword list, something I'm always afraid of.
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We all try to be careful but last week:
For some AdGroups that shot up to the $1, $5, $10 range out of the blue, I decided the most efficient way to deal with the AdGroups I knew would sell big was to leave the list sorted in descending "Default Bid" order.
Then I would start at the top and delete the ones that were highway robbery and raise the remaining so the ads would show (questionable move since I was getting impressions & clicks on "Inactives" anyway).
On one AdGroup, I forgot to delete the outrageous ones and just activated all.
Started paying $10.00 for what should have been 20 cent keywords.
Oh well, live and learn.
Israel