Anscombe's Quartet
en.wikipedia.org
en.wikipedia.org
https://www.autodesk.com/research/publications/same-stats-di...
If you have x/z and y/z and you plot 'em out do not be surprised if there is a corrrelation.
https://en.wikipedia.org/wiki/Spurious_correlation_of_ratios
For examples see chocolate and nobel prizes ;o)
Probably a great statistics book that covers this but it’s also in books that teach machine learning. For example, this book covers this and many other fundamentals:
https://probml.github.io/pml-book/book1.html
See section 2.2.6: Limitations of summary statistics