A Mother's Day cream tea advert has sparked fury for depicting a scone with the jam dolloped on the cream.
The National Trust's Lanhydrock in Bodmin angered Cornish locals by depicting the delicacy in the rival Devon tradition on a Facebook post.
Outraged locals said they were going to cancel memberships over the blunder.
The Trust apologised for the "heinous mistake", adding staff will wear #JamFirst badges to support "a proper cream tea".
About 300 people complained on the post, with some claiming it was "horrifying" and "made them feel sick".
The member of staff responsible for the post "had been reprimanded and marched back over the Tamar" to neighbouring county Devon, the Trust said.
Richard Levy, Lanhydrock's Visitor Experience manager, said it was a "genuine mistake" and the staff member was "meekly crawling around trying to avoid the storm" before returning to work on Monday.
Colin Leggo, from Twitter campaign @Jam_First, said: "It's all about identity, it's like the pasty as well. If people get something wrong that you're so connected with... We love how passionate people can be about it, but the reaction did seem full on."
Stevie Selvey posted: "Wow... Are people really getting worked up about a picture of a scone just cause it had cream on first? How pathetic."
Mr Levy told the BBC he decided to get #JamFirst badges especially made after posting: "There's no truth in the rumour that we're renaming the property Jamhydrock".
Emma Phillips bravely responded "#creamfirstandproud".
Duncan Biscotto posted: "Don't apologise, it still tastes the same - there are worse atrocities in the world."