The phrase "a grave error in" is correct and usable in written English.
It can be used to indicate a serious mistake or misjudgment in a specific context or situation.
Example: "The report contained a grave error in the financial projections, which could lead to significant losses."
Alternatives: "a serious mistake in" or "a critical error in".
So is the NHS making a grave error in supporting this ancient treatment?
The president and Congress commit a grave error in opposing the next round of financing.
In her dissent, Justice Sotomayor said the court had made a grave error in ruling for the church.
They warned that Ed Miliband was making a grave error in joining a lineup of the second division.
I believe The New York Times made a grave error in its coverage of golfer Andy Bean's destruction of a snake during the recent Atlanta Classic (June 21).
Have I made a grave error in planning this trip with my ex, who despite his goodness is a potentially dangerous presence for women?
It will, in any case, be making a grave error if it imagines that Adolf Hitler withdraws before menaces, or gives in to blackmail.
(2) There is a graver error in the Court's holding, however, which may interfere with sound administrative procedure.
This would have been a grave error because, in the words of Sun Tzu, "Do not repeat the tactics which have gained you one victory, but let your methods be regulated by the infinite variety of circumstances".
To take the public option off the table would be a grave error; passage in the House of Representatives depends upon inclusion of it.... [A] final proposal for the President's signature, MUST contain a public option.
This would have been a grave error because, in the words of Sun Tzu, "Do not repeat the tactics which have gained you one victory, but let your methods be regulated by the infinite variety of circumstances". Before I could make that mistake, the customer loyalty representative said she had good news: She could get my bill down to $39.99.
