Ajax有时会出现500错误

I know there have lot of duplicate questions about 500 internal error, but my case is the console "sometimes" shows the error message not always. I'm wondering if I add too many javascript libraries in the beginning of the HTML? I add total 11 libraries files, jQuery at the first, i18n library, and the rest of 9 files. Sometimes console shows one of the file has 500 error, next time shows nothing or another library has the error message. Somebody else have the same problem?