There was some discussion upthread on the various forms of the quote "He who is not left-wing at 20 has no heart. He who is not right-wing at 30 has no brain.", attributed to various figures, but most commonly to Churchill who, together with Mark Twain and Thomas Jefferson, seems to accrue more fictitious quotes than the avergae historical figure.
Lots of people have said similar things over the years, adjusted to the appropriate political conditions of the time, but the oldest known example of the quote is supposed to be from Francois Guizot, a minister in Louis Phillipe's government in the 1830s. His version ran:
"Not to be a republican at 20 is proof of want of heart; to be one at thirty is proof of want of head."
I'm unsure of the actual source for this quote, though, so perhaps that's a misattribution as well.
'Your right to swing your arms ends just where the other man's nose begins' is often attributed to the much over-praised Justice Oliver Wendell Holmes, though in reality it was written by the much under-recognized judicial philosopher Zechariah Chafee who would largely provide the philosophical basis for first amendment case law in the second half of the 20th century.
The very famous quote "Evil prevails when good men do nothing," wasn't really said by Edmund Burke, or any other political philosopher, but apparently just coalesced around the turn of the century, although both Burke and John Stuart Mill did say similar things.
Apparently nobody just wants to end a pithy quote with "-by anonymous."
One of the odder misattributions pops up all the time in people's signatures on emails and forums:
"The world is a book, and those who do not travel read only one page."
- St. Augustine
I've no idea the original source of this quote, but obviously it's not St. Augustine. What I find strange is why someone chose to attribute it to him. It's not like he's one of the typical candidates - why not just make it another 'Mark Twain'?
English schools - at least in my experience - erroneously teach that Julius Caesar uttered his famous "Veni, vidi, vici" upon invading Britain. He actually never said it; he wrote it in a succinct message to the Senate after defeating Pharnaces of Pontus. And it had nothing to do with Britain.