Minecraft 1.4.2, Minecraft 1.4.7, Snapshot 13w04a, Minecraft 1.5.1, Minecraft 1.5.2, Snapshot 13w18b, Snapshot 13w18c, Minecraft 1.6.1, Minecraft 1.6.2
When a URL goes over two or more lines in chat, only the first line will be clickable and clicking on it will open a link composed of only the first line in a web browser.
Multi-line URLs will be copied entirely when clicked on.
Multi-line URLs are broken up when clicked on.
1. Set GUI scale to the largest it will go for ease of reproduction.
2. Find a long URL. (Searching with a long phrase in Google and copying that is a good way to make one)
3. Paste the URL into chat
4. Click on it
5. Observe that only the first line is clickable and used.