expression, noun
common occurrence; everyday thing; not unusual
Used to state that something happens often or is nothing special. Often appears in explanatory or reassuring contexts.
See also: 良くある
日本では地震はよくあることです。
Earthquakes are a common occurrence in Japan.
そんなミスはよくあることだから気にしないで。
That kind of mistake is a normal thing, so don't worry about it.
よくある is the prenominal form ('common ~'), while よくある事 is a noun phrase meaning 'a common thing/occurrence'.
Phrase composed of よく (adverbial form of 良い, 'good') + ある ('to exist') + 事 ('thing'). Literally 'a thing that often exists/happens'.