Combine, and when an exception is still right

When you build one thing out of several independently-validated pieces, you don't want a nest of if (a.IsFailure) checks. Result.Combine takes several Results and gives you one: if any failed, you get the union of all their errors; if all succeeded, you get a success carrying a tuple of the values. It's FailIf's cousin for whole Results instead of boolean conditions.

var combinedResult = Result.Combine(
    Result.Combine(delay3hrResult, delayEtdResult, cancellationResult, crewHotacResult, lostContributionResult),
    Result.Combine(delay3HrsWelfareResult, overnightWelfareResult, cancellationWelfareResult));

if (!combinedResult.IsSuccess)
{
    return Result<FlightDisruptionAggregate>.Failure(combinedResult.Errors.ToArray());
}

var ((delay3hr, delayEtd, cancellation, crewHotac, lostContribution),
     (delay3HrsWelfare, overnightWelfare, cancellationWelfare)) = combinedResult.Value;

So if validation never throws, when IS an exception right? When something genuinely unexpected happens: a bug, a broken invariant, a state the code should be unable to reach. Invalid user input is expected and routine; a Result handles it. A switch arm that should be unreachable is a bug; that throws.

You'll see this line in the real Entity base too: calling Equals on an entity throws NotImplementedException with the message "Don't use equality operators with entities." That's a deliberate, exceptional guard against a category of mistake, not validation. It's the right use of throw.