Complex sentence: The only exception to this is a pub where people will normally lean on the bar to wait but you should be aware of who is waiting ahead of you because it is rude to accept service from a barman before someone who has been waiting longer.
Split into simpler sentences: The only exception to this is a pub where people will normally lean on the bar to wait. However, it is still rude to accept service from a barman before someone who has been waiting longer.