The phrase "clumsy mistake" means a mistake that is made in a clumsy or careless way, usually resulting in some kind of accident, error, or mess. Full definition
Open yourself up and avoid clumsy mistakes during your date. (plusizedating.com)
Due to the lack of experience of the administration they have made clumsy mistakes which look, at first glance, to demonstrate a lack of understanding of the US Constitution, political affairs, legal structure and general all - round US civics. (politics.stackexchange.com)
Showed great alertness to bail Lindelof out after a clumsy mistake from the Swede almost allowed Chelsea a chance on goal. (thehardtackle.com)