Possibly annoyed with myself

Yesterday, I posted two letters. Today I also posted two letters but when I was deciding which stamps to use, I have a niggling feeling that I did not put enough postage on one of yesterday's letters.... putting a first class stamp on instead of a £1.28 (the Great British Films Issue).... I should find out tomorrow... If I have put insufficient postage on, I will be annoyed but hope Royal Mail return the item to me to rectify it.
This will be the first time I've done something like this.... although I have had post returned because a stamp has fallen off (was on a shiny envelope) or when they looked at the back of the envelope where I used to put my address label as sender with them saying insufficient postage (now, I put my address label in top left corner on front of envelope, or write it at the top on the back of the envelope).

Edit - 26th June 2014... no sign of the letter being returned, so perhaps it did have correct postage on. The letter was either off to California or to Minnesota...