If you do this you can get away with practically anything.
If you want to go somewhere you arent really meant to be or do something you arent really meant to do
then if you walk fast enough and look pissed off enough then no one will mess with you.

The last time this worked for me was yesterday morning on my way to work.
For various ( and entirely legitimate - I swear ) reasons I had to catch the train without first purchasing a metcard.
When I arrived at my destination, I was faced with a problem in that the little gates you have to go through wont open without one.
So I walked fast, looked annoyed, vaulted the gate and just kept going.

The eyes of the Ticket Nazi at the gate widened a little but he made no move to stop me or question my actions.

Then again perhaps he was just surprised at such a sudden burst of athleticism from a man in a grey flannel suit.

You can also carry a clipboard and scribble notes furiously - this will get you into most offices and other otherwise restricted areas.

My great-uncle was conscripted in ww-1. So he donned his private's uniform and grabbed a clipboard. Despite orders to the contrary.

He was eventually discovered over two years later, when a Major noticed that he was scribbling and not really doing anything productive.