Thanks to a keen visitor I can confirm that in certain circumstances a pair of full adult peak singles is more expensive than the paper anytime return. The journey must be in a single zone (2, 3, 4, 5 or 6), be priced on the NR1 fare scale (see guide to fare scales) and both directions must be made at peak times (0630-0930 or 1600-1900). There are some quite reasonable length journeys, especially in zones 2 and 6, like Caterham to Purley or Lewisham to Clapham Junction via Denmark Hill. The peak Oyster single is £2.30 making £4.60 for a return while the paper return ticket is £4.40.
For background, before Oyster was extended to cover National Rail in 2010 the single and return fares were standardised based on zones used. You could get anytime singles, anytime returns and off-peak returns. When Oyster started the peak single was set to no more than half the anytime return and the off-peak single was set to no more than half the off-peak return. To encourage Oyster to be used the off-peak return tickets were then discontinued. Since 2011 both Oyster and paper fares have been subject to annual fare increases. The Oyster peak fares have always remained at most half the anytime return fare, until now.