Can you blow a head gasket by pouring water into reservoir tank?

It's extremely unlikely that pouring water into the reservoir tank can cause a head gasket failure. The reservoir tank is part of the cooling system and it holds the coolant that circulates through the engine.

Head gasket failure is typically caused by overheating or other mechanical issues. However, pouring water into the reservoir tank when the engine is running can cause a sudden change in temperature, which in some cases might lead to a cracked cylinder head.