In reply to 5cifi: THat's rather over the top. Often the person serving hasn't noticed, or if taking orders for food is taking a batch of orders before going back to the kitchen.
If in a restaurant, if they take someone elses order 1st and you have been waiting longer, if they then go to the kitchen to give the order rather than go to your table to take your order. Talk to waiter, waitress, say "I am really sorry but we have been waiting longer than ..... can you please take our order". Had to do that once or twice, always got prompt service afterwards.
If in behind the counter service situation and the server hasn't seen whose 1st, you are 1st and someone tries to get served before you, just say. "I am sorry, you must not have seen, but I have been waiting longer, I would like to be served", never failed to work.
If in a pub/bar, if its busy the barstaff don't really have a chance so it usually ends up survival of the fittest though nowing who is going to be served next then asking them to ask the bar staff to serve you next often works.
There you go, no need to teach people a lesson, be abusive or anything else.