Today I used a certain coffee mug for the first time. It has a drink-thru lid. The first half dozen or so times I drank, the flow was minimal and I wondered was the tiny vent hole blocked. Upon examination I saw it was not blocked. Puzzled as to the poor flowage when drinking. Then eureka!, I realized the tip of my nose was blocking the air vent when I drank.
Now I deliberately avoid letting my nose block the air vent and the coffee flows fluently.