just missed a payment by mistake (forgot to update a regular transfer) on a substantial monthly bill. Ouch! Be careful people, be a transactor not a revolver.
If you usually always pay your monthly statement in full, it would be worthwhile for you to call Citibank and explain that you made a mistake and ask if they could reverse the interest charges this time. If you have a good history and have already paid the statement balance now, there is a good chance they will do this for you. I have done it once before with Citibank with success.