You do what ever you need to in order to overcome the situation while avoiding embarrassment to anyone else.  This may mean that you must look like the one who has made the mistake.  Here are a few examples:

•   You discover that the person dining next to you has taken your napkin and you are without. 
  •  You notice that there is now a spare across the table that won’t be used because everyone has one. You can say you dropped your napkin and you see an extra would someone pass it over to you or you could ask the wait staff to bring you one.  Chances are, they will see the spare and bring it to you.

• Someone using your water glass may be a bit more difficult to overcome as anything you do will probably reveal that someone took your glass.  
  • Once again, you may have to ask the wait staff to bring you a glass.  If your lucky they won’t point to the one that should have been yours and thus reveal that someone took yours.  If there is no other way out, you may just have to chock down your dinner and hope they serve coffee with dessert.

Generally, the most graceful thing you can do is call attention to yourself and away from the mistaken party.  Laugh and admit that you just can’t ever tell at these round tables whose glass is whose.  Apologize for the disruption and ask if everyone has the item in question.  This way everyone will claim what they think is theirs and you may get what you want.  Does this diminish your reputation as a lady or expert in etiquette?  If it does, it will be only momentary as you will have gracefully solved the problem possibly to the relief of others who were unsure.