This is a bad form of advice. Instead of "Don't do X", it's much better to say "Do Y instead of X". So in this case, for instance, it's safer to first paste in

This is a bad form of advice. Instead of "Don't do X", it's much better to say "Do Y instead of X". So in this case, for instance, it's safer to first paste into a text editor, examine first, then paste.