About an hour ago, within the space of 26 minutes I received 3 downvotes each on the only question I've ever asked and my 2 highest scored answers.
I don't particularly mind someone I've annoyed in someway downvoting me, but the fact that they've obviously done it with 3 different accounts (what are the odds that 3 people found a problem with an answer I posted over 3 months ago in the exact same minute?) is a clear sign of abuse of the system.
I find it particularly suspicious that https://stackoverflow.com/questions/7221394/what-is-white-space-problem-in-ie, a question I voted to close earlier today, was upvoted 3 times very quickly in that same timeframe, and the person who asked it just happened to post an answer on my own question that was also upvoted 3 times quickly within this timeframe.
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). But professional sockpuppeteers will learn to use different browsers for the different sockpuppets.