If you are in a repository chat room, you can type # and then start typing a bunch of numbers. Naturally, these numbers should respond to an issue or pull request number. You'll notice that when you start typing these magic numbers that a typeahead will appear so you can see what issue or pull request you are referring to.
The output will look like this:
You'll notice that the issue is coloured green, this reflects the status of the item (open).
If you hover over the issue or pull request, you'll see a preview like this:
Clicking on the mention will, naturally, open it in GitHub.
You can also mention issues and pull requests across repositories by typing owner/repo#<number>