Which HTTP status codes cause you the most headaches when auditing a website?

MDN groups HTTP responses into five classes, including redirection messages from 300 to 399, client error responses from 400 to 499 and server error responses from 500 to 599.
https://developer.mozilla.org/en-US/docs/Web/HTTP/Reference/Status
The full answer on what 3xx, 4xx and 5xx codes mean is on the blog:
https://aiwebpageseo.com/blog/what-do-http-3xx-4xx-5xx-status-codes-mean
Which status code problems do you find most often on the sites you check?