General Bernadotte in the French army. Did alot of things wrong, and ignored several orders from Napoleon, which could have ended disasterously sometimes hadn't it been for other skilled French commanders.

Most generals in WW1. French General Jonut/Jounot comes to mind.

Guy de Lusignan perhaps, for marching most of the crusader troops into open terrain, and defying earlier crusader state tactics.

A military mistake, was when six highly experienced Athenian admirals were executed by the Athens Council for allowing some Athenian sailors to drown during a battle. This ment inexperienced admirals and the morale in the navy spiralling due to the executions. Short while later, almost entire Athenian fleet was destroyed at Aigospotami.

Dien Bien Phu. French organization and poor plan of sending in reinforcements.

Fourth Crusade besieges Constantinople. Piss-poor leadership among Greek generals & officers, Greek troops ill-disciplined, low morale & poor training. Byzantine fleet had in addition not been maintained for 8-9 years under Emperor Alexius III rule.
Byzantine Emperors were very quick to flee.