Succes in research, awful in solving in basic things
I have seen my fair share of mathematicians failing basic tasks. Probably more iconic ones are related to arithmetics but that is more understandable.

For example I have seen a +40 years old working mathematician volunteering to teach differential equations because s/he didn't feel confident in his/her ability do solve them and wanted to force him/her-self to improve.

I don't think it is detrimental to fail basic tasks as long as you behave accordingly. As long as you discuss with others while doing your research and prepare before you teach, you can avoid most of the potential issues and fix them along the way.