PG04 : Great ideas for head line copy.

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Don't be a lemon!

 informal an unsatisfactory or feeble person or thing:car-makers cannot afford to create lemons

Don't be silly

Ben: I don't think I'm gonna come to the pub tonight
Rit: Don't be a lemon

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Don't be a bad apple!

a rotten (or bad) apple

informal a bad or corrupt person in a group, especially one whose behaviour is likely to have a detrimental influence on the others:chartered accountants have no time for rotten apples in their professional barrellooks like we hired ourselves a bad apple

[with reference to the effect that a rotten apple has on fruit with which it is in contact]

Informal way of saying "bad influence".

-Tim and Jack are really bad apples for you.

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Don't be bananas!

go (or be) bananas
informal

become (or be) mad or extremely silly:everyone’s beginning to think I’m bananas

become extremely angry or excited:she went bananas when I said I was going to leave the job