Advanced WordPress Developer Overlooked Mistakes

Common nature of being human is to make mistakes, learn from mistakes and correct the mistakes. A lot of mistakes I have made in my WordPress journey so far and learned from them. And I have seen people like advanced WordPress developers making mistakes developing websites, themes, plugins and even customising such as debugging off, using common names for variables, constants, function and classes, not to optimise media, etc. But these are very common mistakes we make. I would like to share the mistakes we actually overlook or do not consider as mistakes.