Look what I found: Slack's free tier pricing page explicitly states "There's no limit on how many people you can add to your team on Slack." https://twitter.com/FreeCodeCamp/status/612758062214950912
Everyone understands that "unlimited breadsticks" implicitly means "lots and lots but not infinitely many"
www.hostgator.com - unlimited disk and bandwidth. Never truly unlimited.
They limit the types of content you can host (no media sites), they set inode limits (http://support.hostgator.com/articles/pre-sales-policies/rul...), they will suspend you if you use too much server resources. Pretty much anything to keep you from using "unlimited" resources.
There are a ton of other examples. I would check out www.webhostingtalk.com if you want to see all the issues with these type of hosts.