1. You start working on your system without a 100% complete plan
2. You start working on your system without all the parts. Sometimes if you wait to get parts, that particular part may be out of stock or completely gone before you know it.
3. You are just a little bit confused on a topic, or system. Make sure you research every single detail you can find about a particular system. If you don't, you could accidentally destroy a component on the motherboard of your system.
4. You're new at something and you don't practice on something first (Ex. soldering or cutting). Again, you can seriously mess something up.
5. You cheap out on parts. Cheap parts = bad portable.
6. You try a new product without a backup or knowledge on the new product. Sometimes trusting new things like a screen without prior knowledge or a backup can ruin your system. New products always have glitches or problems and they could end up bringing down your portable.
7. You have a very messy workspace. Always give yourself enough space to work. Also, it's not a good idea inhaling solder fumes/smoke.
How do I know all of this? First hand experience
