When onboarding and probation collide
A probation period is a decision point, not a filing exercise.

In most companies the probation review is a calendar reminder that fires late, gets confirmed by email, and means nothing. That is a waste of the single most useful decision point in the employment relationship.
Know which system you are in
In much of Europe, probation is a defined legal period with shorter notice on both sides, statutory maximums and specific paperwork. Miss the deadline and the protection lapses automatically. In most of the United States employment is at will and probation is a management convention rather than a legal state, though it still shapes expectations and unemployment claims. Canadian provinces sit in between, with statutory minimums that bite early.
Review at the midpoint, not the end
A single conversation at the end is a verdict. A midpoint conversation is a chance. If somebody is struggling at week six of a twelve week probation, they can often be turned around with clearer expectations or a different pairing. Discovering it in week eleven means the only options are extend or exit.
Write down the evidence as you go
Managers who decide at the end reconstruct a narrative from memory, which is where bias lives. A few short notes each fortnight against the ninety day plan gives you an actual record, and it is what protects the company if the exit is contested.
Be willing to use it
A probation period that never results in an exit is not a standard, it is a formality. Equally, an exit at probation is nearly always a hiring or onboarding failure rather than a person failure, and it deserves a short honest post mortem: what did we miss, what did we oversell, what was the role actually like.
Close it explicitly
When someone passes, tell them clearly and in writing, and say what changes. A quiet expiry leaves people uncertain for weeks about whether they made it, which is a strange way to begin.