One of my users bought an account upgrade (yearly renewing subscription) which is due to expire in 2015. For some odd reason they bought the same upgrade a month later and xenforo has rightly extended the expiry period to 2016. The user then explained their error and asked for a refund of the latest payment which I processed. The initial payment was not reversed, only the second (most recent) payment.
I expected xenForo to knock the expiry date back down to 2015 but it actually cancelled the entire subscription.
I'm not sure if you'd class this is a bug and I'm not sure if it can be coded around.