Even the most experienced users make mistakes and sometimes we leave an inner face here, a hole there, so there’s all sorts of addons that check your mesh for errors. I used to use one called Mesh Lint, but that was a long time ago. I have it on my to do list to buy a good addon that checks my mesh, just to tidy up things at the end. I think Blender has some built in tools or addons that do this, but I have to double check on that. The thing is, those mesh checker addons helped me a lot when I was learning Blender, you begging to train your mind on what to start looking for when the addon points it out for you, since at the beginning you you’re still not sure what mistakes to look for.
