1. The myth of the "straight back." Obviously, you don't want to do squats or deadlifts with a big curve in your back, either concave or convex. But you do want to keep a small natural lumbar curvature in both squats and deads. This helps keep the body as upright as possible instead of bent over and stressing the back more.
You can take the curve out of your back. Just tuck the coccyx under your spinal column instead of pointing it backwards. Practice flattening the lumbar back against a wall. But while this does "keep your back straight," it is not generally recommended. Your back is actually stronger with a slight concave curvature. Of course your mileage may vary if you have back problems.
For years I tried to "keep a straight back" when lifting weights. I think it was a mistake.
2. The myth of wide grip bench presses. You have doubtless heard the advice: "Grip wide because it works the pects more." Well, gripping wide causes a lot of injuries, even under light weight. If you grip wide and keep your humeri pointing out at 90 degrees from your body -- yes, that does hit the pects but it also is hell on the shoulders.
Part of the problem is that with various exercises we are told to "stand with feet shoulder distance apart" or "grip the bar with hands at shoulder distance." This brings in the ego: Nobody wants to have narrow shoulders, right? And however wide or narrow our shoulders are, body image tells us they are wider than they are. So we go around thinking our shoulders are a lot wider than they are. It is not uncommon for us to turn sidewise to go through a door that is quite big enough. The influence of body image. False body image messes with the judgment.
In the case of bench presses, biomechanically, the grip should not be wider than 1.5 times shoulder width. But "shoulder width" refers to the width between shoulder joints and not measured from the outside of the medial delts! In my case "shoulder wide" is about 13 inches!! So I ought not to grip the bar any wider than 20 inches! Ego would have me measuring my shoulders at 20+ inches and taking a bench press grip with hands 30 inches apart, and I think it was using a 40 inch grip that injured my shoulder. Beware of ego issues and false body image.