Of the seven mistakes I have made some of them. Maybe not at first a mistake but over time things change.

Thin width of a sleeping bag? Yes. I roll around a lot. Often end up with a frozen arm and shoulder outside on the ground. Lying with the bag turned around.

Same with a too narrow mat.

Funny a local sambar hunter once recommended a two-thirds length mat. Said full lengthnis not needed ie for the legs. Saves weight and bulk backing. No I disagree.

I once used one mattress, one sleeping bag and one blanket camping in France. Two people. Woke up in a lot of pain and stiffness from the cold from the ground. We couldnt find decent stuff in London to buy. So took what I had till we reached Germany where decent camping gear could be bought. Back in 1989. Bad decision. Several aweful nights without adequate insulation below. With a mat and sleeping bag each heaven!