Wimbledon 1 Derby 0
Darlington's back-heeled winner was the reward for the dominant Dons against Derby, who secured their safety the previous week.
But Wimbleon were grateful to keeper Scott Bevan for an early save from a header from Junior.
Wimbledon responded as Dean Lewington headed over and Melvin Kamara first shot wide and then drew a fine stop from Lee Grant.
The Derby keeper also saved from Wade Small while Adam Bolder kept out Wade Small's goalbound header before Darlington's cheeky finish from Small's cross.
Mark Williams wasted a chance to double Wimbledon's lead before a Derby fightback in the second half.
Bevan saved twice from Ian Taylor and also denied Junior, Marco Reich and Marcus Tudgay as Wimbledon won at home for the first time in six months.