The phrase "actually a mistake" is correct and usable in written English.
It can be used to clarify that something is indeed an error or misunderstanding, often in a conversational or explanatory context.
Example: "I thought I had submitted the report on time, but it was actually a mistake; I forgot to hit the send button."
Alternatives: "truly an error" or "indeed a blunder."
The cheese was actually a mistake.
So we have cobalt, two waters -- three waters -- that's actually a mistake.
I'm not sure that's the real reason; I think it was actually a mistake.
But Mr. Blaine, who was inducted into the Rock Hall of Fame in 2000 and the Percussive Arts Society's Hall of Fame in 2012, said the beat was actually a mistake.
Notice that a conversation between me and someone who said that would stop far short of deciding whether the war was actually a mistake and immoral; the focus would remain on the tactical landscape and not on questions of morality and justice.
That "intellectual strategy" is actually a mistake, which is why mainstream economists miss the importance of banks in creating money.
Surprisingly, past research shows that there is actually high satisfaction with primary care doctors in Hong Kong, and that the perceived prevalence of doctor shopping is actually a mistaken myth among Hong Kong people [ 30].
One day, perhaps, he might actually make a mistake on the horse.
Hiram Sasser, a lawyer for the Liberty Institute who represents the King Street Patriots, denied the claim but when presented a video of the incident, he said that his client had actually made a mistake and did not realize the office was tied to Houston Votes.
The police officer actually makes a mistake and seeks to absolve himself of blame by passing the responsibility for the mistake on to Thomas.
Vettel actually made a mistake in the second session on his first lap, forcing him to have to take a second set of tyres for a second lap to ensure he progressed.
