MySQL Will Quietly Eat Your Data If You Let It: Truncation, Charsets, and the 1366 You Never See

A varchar(10) column silently chopped three digits off a phone number and nobody noticed for months. Non-strict mode turns bad writes into warnings, strict mode turns them into 500s, and utf8 in MySQL is not UTF-8. The queries I run to find truncation and mixed charsets in a database I inherited.