When copy and paste just aren't enough
In my job I often send my customers detailed instructions including things to run at the command line. Sometimes they don't work even though they look correct.
The plumber's mate has written a very helpful post that explains the problem that is often the explanation of why this happens.
Short answer is - if it doesn't work with copy+paste, try typing.
The other gotcha that often comes up is if, for example, you email someone a password and they select it by double clicking on the word (in Outlook) and both the password and a white space after it are selected. So when you copy it you are including a white space that is not really part of the password. But the person using it does not realise because when they paste the password into the password box the space becomes an asterisk.