The fear of missing something is real. I have seen decisions taken in Slack by two or three of the four or five people who should have been involved. I'm not saying this is caused by Slack -- the problem is absolutely with the people who thought it was okay to act on such a decision. But I do think it's facilitated; the people involved seem to subconsciously think that because they're discussing in a public channel where the other people are "present" -- even though they're not active -- that everyone's informed. Whereas if they'd just had a hallway face-to-face, someone would have said, "Hey, we really need to check with Laura and Ed about this too".