I was trying to create a reddit account but didn't want to use my real IP address for it. So when I was trying to find a working proxy I kept refreshing the site to test each one, then when I found one it told me they have been getting too much requests from my computer and I had to wait like 10 minutes before I could sign up. How did they then know that all the requests were being made by the same PC when I was behind a proxy? I even tested the IP to check that it was different from my real one and it was.