A mixed redirect is when there are 2 or more other URLs redirects strung together in a chain, but the HTTP status is not the same for all redirects. At least one redirect in the chain is a 301 permanent redirect and at least one redirect in the chain is a 302 temporary redirect.
Both a 301 and 302 redirect are used for different purposes, and each sends a different message to search engines. If they are both used in the same chain, search engines will not know how to treat the URLs – "Is it actually a temporary redirect or a permanent one? Should we remove the URL from our index or leave it? Do we pass link juice to the target URL or not?" Therefore, we cannot predict how search engines will treat these URLs, and unintended consequences could result. Therefore, mixed redirects should be avoided.