Too much or too little energy when baking (burned or raw in the middle), too much or little water (hydration), too much or too little rise time (loaf density). Just off the top of my head.

Adding incompatible ingredients (like honey) that mess with the yeast culture or kill it. "Peeking" at you loaf during bake too often or for too long. If you use a home bread machine, layering the ingredients incorrectly so that different things start too soon.

An important exception to "Always PEEK before you POKE.", because peeking can change the value of what you eventually slice.

. o O ( Almost wrote ".slice". And I'm sure there's a tortured Heartbleed/Spectre/Meltdown analogy in this loaf somewhere… )