The pessimistic misconception that "it can't be done."
If you are mistakenly thinking "be made" when in fact you can't, you have a chance to try it and experience that it's surprisingly not possible. If you mistake "I can't." for "I can't" when in fact you can, you don't have a chance to experience "surprisingly, I can." because you don't try. The former mistake is corrected, but the latter is not.
So the latter mistake is more serious.
If you don't know which one it is, you should knock it down to the former.
---
This page is auto-translated from /nishio/「できない」という悲観的な勘違い. If you looks something interesting but the auto-translated English is not good enough to understand it, feel free to let me know at @nishio_en. I'm very happy to spread my thought to non-Japanese readers.