1. Don't let my beer glass touch the faucet nozzle. Jamming the glass into the curve of the faucet is the mark of someone trying to work too fast. A pint pours in eight seconds from a properly functioning draft system. Who needs to work faster than that? In addition, when any contact is too hard, the glass can chip. No one wants to drink a beer that may or may not include a shard of glass. There are cleanliness issues with this as well (see my next point).
2. Don't let the end of the faucet dip into my beer at the end of the pour. it's not sanitary. A faucet nozzle immersed in beer becomes a breeding ground for microorganisms, possibly infecting the line and contaminating all beers until that line's next cleaning when the faucet should be completely disassembled and hand detailed. A faucet nozzle transfers contamination to every clean glass it touches after the immersion.
3. When serving my beer, feel free to touch anywhere you need to on the bottom two-thirds of the glass; that real estate belongs to both of us. The top third of the glass is mine. I'm going to put my mouth on that, so please don't pass my beer with your fingers clasped around the rim like a construction crane lifting something into place.
Whenever I see these mistakes being made, I see a training issue. If people are trained the correct way to do something, they'll almost always do it correctly. Avoiding these simple service mistakes will elevate beer. If you are a bar or restaurant owner wanting help with these and other training issues, I am a Certified Cicerone® for hire; feel free to contact me.