Last year we switched to using Slack for all our internal communication and it's working out nicely. It's very developer centric in that it offers integrations with lots of services like Travis CI, GitHub, etc.
When we started using Slack one of our developers was sending a file, had his Developer console open and noticed that even though he'd not chosen to share the file public, the API gave back a public URL anyway. Much to his dismay when he tried it out in a new private browsing window he could download his file without authentication!
Everything you share on Slack automatically becomes available on a public url.
Concerned with the security of our communications (we don't share financials or credentials through Slack fortunately, but we may share company or customer sensitive information) I decided to look into it and make it a teachable moment on 'secret URLs'.