Year 2
Dumbledore dosen't take any notice that he's sending harry back to the abusive horror house. When the year starts, he completely disregards that Harry and Ron miss the train, looks nothing into it and finds it completely normal.
He hires Gilderoy Lockhart, a man very clearly incompetent at everything. A single minute in his presence makes that abundantly clear. It's claimed that he was the only person for thre job. Okay sure. But where the crap were Lupin and moody and all the past DaDa professors? Dos they just like pop up from Tartar?
He was at Hogwarts when the chamber was opened the first time, for the most powerful wizard, he sure is dumb for not realizing that Slytherin = snakeCat = petrified Myrtle = dead and unscathed. No trace of poison (Hotel= Trivago lmao)Other than the killing curse, I believe a basilisk is the only way to be killed and still be unscathed and the death cause unidentifiable. (Other than poison). And if he did know why did he let Harry and Ginny go through that.
Veriteserum could have been used to make Lucius Malfoy confess that he gave Ginny the diary, but nope just let him get away with it I guess.
Dumblebleep couldn't have just used veriteserum or legilimacy on every student to find the heir of Slytherin. 'Lol forgot, sorry Harry, at least you didn't die'