My IP is banned in 4chan for months now, and I didn't even post there.
I tried the wifi in a relatives house, which is the same ISP, and the IP is banned, too.
I guess they banned a whole fucking range of IP, which is very stupid...
I was once doing something with a friend, nothing bad I assure you, but the site in question did no approve of the loophole we found... nor its consequences, so they banned our accounts. Basically, and without giving too much detail, if two people used the same ip to access the same "chat" and "talked" to each other... the site did not like it.
At first we reacted as adults, since it was a reasonable reaction as the issue we accidentally found slowed the site down to a grind(roughly, buffered images from established connections kept coming in, but were going out at 1f/s, and you could send a new successful request every 2 seconds more or less), and tried to reason with them that it was an honest mistake and we would like our accounts back. It did not go well and they were not reasonable.
So we, not responsible adults according to them, acted as the childs we apparently were, got new a new IP and made new account, so every few minutes they would find us(because loopholes are there to be exploited) and ban the ip.
It took us a whole 10 seconds to get a new ip and under a minute to be in the site again. We went through many, MANY, IPs. They had to freeze the account creation system in the end to stop us. Sadly they fixed the issue eventually.
It was a fun summer's break afternoon.