Hitler made mistakes later. One of the most serious was invasion into the Soviet union.
It was a mistake to do so without nuetralising the UK
It could be seen as a mistake to do so in Jun 41 before the losses of the BofB are made good
You can argue the vBoB was from a German perspecteve a pointless waste of combat aircraft and crews
You can argue the whole Desert war was an unnecassery diversion of resources (especially trucks)
But you cannot call the invasion of the Soviet Union a mistake* since that was the whole point - If Hitler doesnt do that he isnt Hitler
*Bad Idea - poorly executed and unwise perhaps but not a mistake