Fairness requires consistency, but wisdom requires context.

A standard that shifts with the wind is not a standard, and a standard applied without regard to circumstance produces absurd results. Both halves are true, which is why this is genuinely hard rather than a matter of choosing the right principle. The resolution is not a formula; it is being consistent about the reasons while allowing the applications to differ.

When you make an exception, make it on a stated ground that you would apply to anyone in the same position. Write the ground down if it matters. That way the flexibility is part of the standard rather than a crack in it, and other people can predict what you will do.