What is the difference between an innocent mistake and misrepresentation under immigration law?

The line isn't really about whether the applicant meant to deceive anyone — it's about whether the incorrect information was material to the decision and whether it was reasonable, in the circumstances, for the applicant not to have caught it. A truly minor slip on an immaterial detail, or an error that a reasonable, careful applicant genuinely couldn't have been expected to notice or correct, may be treated as an innocent mistake rather than misrepresentation.

But this is a narrower category than most people assume. Simply not intending to deceive anyone isn't enough on its own; officers and, where it gets there, the Immigration Appeal Division look at whether the applicant exercised reasonable diligence and whether the information mattered to the outcome of the application. A significant factual error that the applicant reasonably should have caught — by reading the form carefully, or by knowing their own history — is generally treated as misrepresentation even if there was no intent to mislead.

Because "I didn't mean to" is rarely a complete defence on its own, anyone who realizes an application contains an error, however unintentional, should get advice about correcting it properly rather than assuming good intentions alone will protect them.