Living in NYC - the land of "no notes that we are out of X" - the only way a note would make someone look "incompetent" is if there was a spelling error on the note IMHO. I would much rather see a note on the door than waste time waiting in line only to be told "we're out". (This not only applies to Starbucks up here - I have also waited in line at McDonald's for a McFlurry (obviously before my gluten-free days) only to be told "We're out of ice cream - but we've got apple pies 2 for 1!" Yeah...I wanted something COLD.)
We all understand that things happen! (Reminds me I need to call the Kips Bay theatre to see if they ever got the special lanyards in that got delayed by the snowstorm.) But IMHO making people wait in line and then telling them makes the place look more incompetent. But that's probably why I'm not management.