Behold the Brostika! Like the hidden arrow in the FedEx logo, Slack's new brand has a negative-space surprise in store. The bars point the "lucky" left way, at least, rather than, well, to the right.

Here it is with the negative space filled in:

Below are Slack's "Logo explorations for the octothorpe." Octothorpe is a fancy word for the pound/hash symbol, which they have somehow managed to turn into a contact sheet of procedurally-generated swastikas.

UPDATE: T. Carter Baxter nails it better than I: