Many I'd have thought are pretty universal... some are just plain daft. Anyway, here's a selection of the whackier faux-pas listed at Wikipedia - List of faux pas.
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The North American custom of eating with just a fork is considered bad table manners.
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Letting one or both hands rest under the table or on your lap during eating is considered bad table manners
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When clinking glasses you are supposed to look into the person's eyes who you are toasting. Not doing so results in seven years of bad luck (or, more specifically, bad sex).
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German pubs (especially less modern ones) commonly feature a bell hanging over the counter, facing the customers. Don't be tempted to ring it, unless you intend to pay for a round for the whole bar.
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Putting your glass down on the table after clinking glasses (and before drinking) is considered rude in some parts of Germany. It is said to "invalidate" the "Prost" (Cheers). One exception to the rule is in southern Bavaria, where it is customary to put your glass down on the table briefly before drinking.
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Especially in the north of Germany, using a candle to light a cigarette is said to kill a fisherman.
