"most egregious mistake" is correct and usable in written English.
You can use it to describe the worst or most severe mistake someone has made. For example: "Jeff made the most egregious mistake when he forgot to submit his project on time."
Perhaps the most egregious mistake I found was a single alteration made to the original Coptic.
Graham's most egregious mistake occurred when he muffed a punt and lost possession for the return team.
But the most egregious mistake occurred on their way to the market, when troops fired on a speeding pick-up truck as it tried to pass the convoy of Humvees and troop carriers.
Honoring Dr. Egas Moniz of Portugal with a Nobel in 1949 for discovering the therapeutic value of a lobotomy in certain psychoses is perhaps the most egregious mistake ever made by a prestigious biomedical awards committee.
At the same time, he accuses other women of committing a laundry list of typical female mistakes he details in chapters with subheadings such as "Taking Things Personally," "Making Excuses" and, in a description of perhaps the most egregious mistake, "Not Keeping Secrets".
In response to angry reactions from colleagues, Smith apologized and called the episode "the most egregious mistake" of his career.
Even though it hurts to admit it, the truth is, the most egregious mistakes in Madden NFL 12 were my own.
It was a measure of his miserable game that his four penalties, which went for 46 yards, were not his most egregious mistakes.
It feels a lot like an upmarket Evo, in that the S4 always flatters your driving skills, its trick all-wheel-drive system compensating for all but the most egregious mistakes.
There have been countless misuses of QR code technology, but some of the most egregious mistakes have resided in not giving people a discrete action to take upon scanning.
"I made what can only amount to the most egregious error of my career," he said.
