Just due to the random nature of genes and that processes just go wrong (assuming no allogenic, or outside, factors, like alcohol, radiation, etc.).
Have you ever gone somewhere repeatedly, done by the same person and asked for the same thing (think fast food restaurants)? When you have gone there, have there been times where they get the order wrong? That’s exactly how it goes wrong – just random chance (again, assuming no outside influence or underlying conditions that make mutation more common). This is cornerstone that drives traits to be placed under selective forces for evolution.
Edit: removed redundant words
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