Finding a loophole is clever. Using it to hurt someone who trusted the rules is not.

Spotting a gap in the rules is genuine skill, and it tells you nothing about whether using it is fine. Rules exist because people organise their lives around them, so whoever gets hurt by a loophole is usually the person who read the rules and believed them. Clever and decent are separate questions, and only one gets settled by finding the gap.

Ask who was relying on the rule as written before you use what you found. If the answer is a person rather than an institution, say what you spotted instead of using it. Where it is harmless, use it and be willing to explain it out loud.