Forgot to tap out?
You can get the extra fare back. TfL refunds most incomplete journeys by itself, so wait 48 hours. If yours isn't refunded, ask through your TfL account within 8 weeks of the journey. TfL lets you ask online for up to 3 a month; for more, call TfL on 0343 222 1234.
Sources: TfL: incomplete journey refunds (opens in a new tab); TfL online account terms (opens in a new tab)