Most people should pay extra attention to #7. E.g. Goodfellow et al did even worse than this sin in the Deep Learning book when they claimed the condition numb

Most people should pay extra attention to #7. E.g. Goodfellow et al did even worse than this sin in the Deep Learning book when they claimed the condition number for a square (but not necessarily normal) matrix is defined in terms of eigenvalues. This is false, but nevertheless see 4.2 in https://www.deeplearningbook.org/contents/numerical.html https://www.deeplearningbook.org/contents/numerical.html . When I've raised this with people in real life, I typically get some reflexive response that it should be a useful approximation, but as this blog points out, that isn't true either.