lonelytraveller wrote:
When oyster card season tickets expire, it still lets you through the
gates for one journey the next day. But it charges you pre-pay, and
even when you renew the season ticket, you still have to pay the
pre-pay fee. So that day cost you twice the amount it normally would.

Is there any way of avoiding this, or getting your money back?

Yes. Look at the little warning that says "ticket 1 near expire" or
set up email warnings from the oystercard website.