彼にはよくあることだが、彼は簡単な間違いをした。

English Translation

He made a careless mistake, as is often the case with him.

Robotic word-for-word translation. We don’t repeat 彼 twice in the same sentence like this.
Also, I don’t translate “careless” as 簡単な in this sentence structure. The literal meaning of 簡単な is “easy”, but 簡単な間違い can be interpreted as either 1) “small” (and not so critical) mistake; or 2) “careless” (or “easily detected by others”). It’s vague. Moreover, we often insert another word between 簡単な and 間違い (e.g. 簡単な計算間違い = mistake in an easy math calculation), but 簡単な間違い as one phrase is used less often.

Here is my alternative translation:

よくあることだが、彼は凡ミス(を)した。(pronunciation: 凡ミス = ぼんみす)

凡ミス stands for 平凡な(へいぼんな)+ミステイク (loan word from mistake). 凡ミス is a common word. It sounds a bit casual, but newspapers also use it.