Translated excerpts from a PostShow blog post entitled "the 12 Taboo Ways of Using Chopsticks":

1. 三长两短 Three long two short [an idiomatic expression that translates to "an unexpected misfortune"].
This refers to before or during meals, when a pair of chopsticks is not neatly put together with its ends matching. This is not lucky...

9. 定海神针 The Needle That Stopped the Sea
Sticking a chopstick into the food in a bowl, this is no good, this is understood to be humiliating towards the others at the table. Doing this during a meal is like sticking up your middle finger within a group of Europeans, this is no good.

Link to original post with all twelve ways!

P.S. Image from ShanghaiBlog: four german students in shanghai.