When interpreted under most character encodings, the following line appears to contain gibberish, but note that it doesn't contain any characters that are 'special' in HTML (apart from the <p> and </p> tags at the start and end if you do a 'View Source'). But if you set your browser to interpret this page in UTF-7 you'll find that the line DOES contain HTML tags. Now think what could have been achieved if they had been script tags...
Just to say +ADwA-b+AD4A-BOO+ADwA-/b+AD4A-!