If a plan only works when everyone does their part perfectly, it’s a wish. Build in slack.

Perfect execution by five people is not a plan, it is a coincidence you are relying on. Somebody will be ill, somebody will forget, the bus will be late, and none of that is unusual. Slack is what absorbs the ordinary amount of human failure. A plan with room in it survives the week it actually has to live through.

Find the single point of failure in the next group thing you depend on. Add one buffer: an earlier deadline, a second person who knows how, a version that works with a piece missing. One buffer in the right place beats three in the wrong one.