This is the most lucid explanation of aliasing I think I've ever read.

I mostly deal with this stuff in the realm of audio where the matter of analog vs digital rages regularly. The reason one hears "Nyquist says" so often from people defending digital audio is that the pro-analog people are imagining stair-stepped signals in which the information that is "lost" results in a noticeable degradation in sound quality. This (11-year-old) video is the gold standard for addressing this concern:

https://www.youtube.com/watch?v=cIQ9IXSUzuM

Curiously, no one ever seems to mention the actual problems (like aliasing).