I accidentally poured water into the engine coolant reservoir and wondering what kind of damage I’ve done if any?
If your coolant was at the proper ratio a little water in the reservoir will not make much difference. But I have to ask, what did you intend to do with the water in the first place ? If you meant to fill the wind shied washer tank why were you not using washer fluid?
No damage at all. Coolant IS 50% water and the rest anti-freeze.