The one I saw was this one,
a copy of which is now posted at my desk:
Which made me laugh the most because one of my peers at work has almost the exact same postcard, except hers is intended to be motivational. The contrast made me laugh even more because unless you read the saying below the big bold-lettered word, you'd never know it was a put-down instead of something inspirational.
So here are a few of my favorites. I hope no one is offended, but if you are ... I'm sorry! They still make me chuckle and you can just go ahead and think less of me for it.