In an argument (usually a political debate), a concern troll is someone who is on one side of the discussion, but pretends to be a supporter of the other side with "concerns". The idea behind this is that your opponents will take your arguments more seriously if they think you're an ally. Concern trolls who use fake identities are sometimes known as sockpuppets.
In the 2006 election, an aide to Congressman Charlie Bass (R-NH) was caught concern trolling the opposition on local blogs. While pretending to support Bass's opponent, Paul Hodes, the aide argued that Hodes couldn't win because Bass was an unbeatable candidate. Hodes won the election.