It's not usually a problem though.
So I get alerted to the fact today that our site is pretty much invisible to Google. It's nowhere to be seen in search results.
We find hundreds of thousands of 403 errors on Google Webmaster Tools and we investigate a bit further.
Turns out that one of our IP bans (the aforementioned user) had been spoofing his IP address. The IP address he was spoofing with was Google's.
We had IP banned one of Google's crawlbots.
This has now been rectified. And we will never ban an IP address again.