The highest-value slot, and the one that exists only if someone writes it deliberately — because by the time you know these things they feel too obvious to mention.
The shape is always the same: a reasonable person, acting sensibly on the information available, does something that goes badly, and nothing in the environment warned them. The service that looks decommissioned and still serves one important customer. The config regenerated nightly, so edits vanish with no error. The dashboard everyone trusts that has been wrong since a migration. The person outside the approval chain who escalates when not consulted.
This is the residue in the gap between 35 days to basic productivity and three to nine months to full. You cannot learn a landmine from documentation, because documentation describes what is meant to happen; a landmine is a place where what is meant to happen and what does happen have quietly diverged. The only two ways to acquire one are to be told, or to step on it.
Each note: what the trap is, what it looks like from outside (which is why it catches people), and what to do instead. Where a landmine is genuinely fixable, say so — some survive three role-holders precisely because each one learned to route around it and never wrote it down, and a note is often what finally gets it fixed rather than merely survived.