Mistakes get made in the heat of battle; however the general consensus I've seen is that the battle of Monte Cassino could have been avoided if the allies had moved more swiftly from landing.

It's worth noting that Montgomery allowed the exact same mistake to be made in normandy when the British landed. The Q plan required that they move inland immediately towards Caen, but instead the troops on the ground thought the objective was to secure the beach and, in any case, lacked the armour necessary to proceed inland., Due to that, Caen ended up getting plastered just as the monastery did.

keep to the Fen Causeway