It would depend on the issue. Clothing "malfunctions" then stranger or not I would and have taken them aside or walked up to them and quietly informed them of whatever the issue was. Health issues, such as a nose bleed, I'd do the same if they seem oblivious to it and continuing to just wipe your face would indicate that. Hygiene issues are always more difficult but once it's been raised they benefit too. Never yet has a person I've told of an "issue" been ungrateful, be it clothing, hygiene, or whatever - it's how you tell them and the tact you employ, not what you tell them.