frighten the horses cause consternation or dismay; shock.

☞ The expression has been attributed, not altogether reliably, to the English actress Mrs Patrick Campbell (1865–1940), who allegedly said, in reference to a male homosexual affair, something along the lines of 'My dear, I don't care what they do, as long as they don't do it in the street and frighten the horses.'

2011Guardian Did any performance look less than 100% mainstream? I mean don't frighten the horses or say bloody, anybody, otherwise Lauren 'Culture Vulture' Laverne will be jolly cross and not plug your new album.