What grammatical errors really piss you off?

[paperjam]

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I absolute hate it when someone says 'less' when they should be saying 'fewer'. At least 80% of the population does it. Example: "there are less calories in fatass food A than fatass food B"

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Heck yeah! I used to correct people on that one, but it became impossible because people did it too often, so now I just cringe every time I hear it.