To follow on the good advice that Tim posted, airline lounges where there are free computer stations (i.e. most of them except for UA!) are a particular problem to be mindful of. Clearly quite a lot of folks using these locations are FT'ers.

I have read many times where FT'ers have logged onto to FT at lounges, where the previous person to have do so's user name and password spring to life.

I saw a post on FT once where someone had typed:

"I am not xxxxxx and I have not ever visited this board before. However I suggest NEXT time that xxxxxx closes down the screen before he leaves the computer."

There was no harm done there, but that person clearly if they were minded could have changed user profile data, or posted some really nasty stuff.