I'd add botching the followup to El Alamein, letting Rommel escape intact to Tunisia, screwing up at Caen, Halting his troops at Falaise, not clearing the approaches to Antwerp...
Botching the followup to El Alamein, letting Rommel escape intact to Tunisia
Well I would say that is like criticizing your football team for beating your opponent 6 nil when you think it should have been 7 nil - it was still a massive win - part of the reason was that Montgomery did not trust his armoured forces to run off and get themselves shot up...again.
screwing up at Caen
Well the plan didn't survive contact with the enemy (I blame the Germans) - but that being said the actual battle for Normandy was over far far quicker than planners had expected or anticipated (I blame Montgomery)
not clearing the approaches to Antwerp
Well here's the thing - Antwerp fell on the 2nd Sept after a spectacular and quite unanticipated advance by Br 2nd Army mere days after German Army Group B had only just retreated across the Seine effectively ending the Battle of Normandy (Operation Overlord) on the 30th Aug.
Basically the British were not in a position to clear the Approaches before their supply train had caught up and the specialist vehicles and equipment necessary to conduct ops in the area - they had quite spectacularly outrun their supplys
With hindsight maybe the op could have been started in late Sept instead of Operation Market Garden -but that would have left a large area to the East of subsequent Scheldt operation in German Hands which in itself may have further delayed the Scheldt Estuary anyway.
All that being said I doubt very much that the Battle to clear the Scheldt could have started much before the 2nd Oct 44 and the operation to clear Walcheren Island required a lot of specialist equipment and the units that would conduct the seaborne assaults
Many of those units had been in action from D-Day till Early Sept (No4 Commando for example was in action for 82 days - had 14 days RnR back in the UK before training and absorbing replacements for this op)