“Happens” is the correct spelling of the word. “Happens” is used with the third person singular for the verb “to happen”.
This type of stuff happens from time to time.
“Happenes” is a misspelling of the word “happens”. You should not use “happenes” in English.
We use “Happens” with the third person singular for the verb “to happen”. Happen” is a strange verb because we do not usually use “happen” with human subjects. For this reason, you can usually use “happen” with singular subjects and “happen” with plural subjects.
It happens sometimes, get over it.
These things happen sometimes.