These are off the top of my head so I am likely missing some:
Not buying a back-up striker
Paying $60 million for ndombele
Paying $30 million for Sessegon
Not having a back-up RB or replacement Rb for Aurier
Not having a replacement LB for Rose or other option than Davies for LB
Not selling Eriksen soon enough
Buying Janssen
Buying Soldado
Buying Nkoudu
Buying njiang
Not paying to get Grealish
countless mistakes with the building of the new stadium\
furloughing workers
Juande ramos
Spending of the Bale money
Tim Sherwood as Manager
AVB
Louis Saha
No transfers during the summer
No transfers during the winter
Starting Sissoko
Signing Sissoko
Repeatedly not taking EL seriously
Repeatedly not taking League Cup seriously
Repeatedly not taking FA Cup seriously