Need help with reference lettters!!

[Rock]

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for Reference Letters, I am just wondering what could be examples of 'weakness' that the referees could write..A common one I have heard is the 'quest for perfection'...any other examples?? my referees have actually asked me about this, as they don't know what is considered too bad, and what is considered good that is made to look bad...
Thank you to all those who help with this