Up until today I thought humanity was all on the same page about this, but let's just clear this up right now. If you are on a public bus and:
It's the last bus of the day so your fellow passengers can't transfer off.
You have no formal musical training.
You are with 8 friends who also have no formal musical training.
You only know hymns.
Then it's inappropriate to sing a capella for 30 minutes. You can use the context clues around you to really decipher this such as:
You notice a quiet auction in the back as people vie for emergency exit seating.
The fellow passengers immediately start discussing the pros and cons of hitch hiking and you hear someone say "it's worth the risk".
Everyone on the bus except for you is plastered against the window as if they are trying to put as much distance between you as possible.
Families break apart into groups of "can and cannot walk 12 miles"
A girl's ipod battery runs out and she frantically tears through her purse trying to assemble a backup generator from a granola bar wrapper and a hair clip.
On an unrelated note: If you are in a car with a female who even with fair to middling self esteem knows she is 3000 times better looking than you and:
There are no lulls in the conversation
She is not flirting with you
She has never expressed any interest you
The topics at hand are not beauty, dating, or likes and dislikes or "what would the biggest jerk in the world say right now?"
Then it is inappropriate to say to an obvious brunette completely out of the blue that you only find blonds and redheads attractive. What's your common sense primer of the day?