What does Oberon realize when he sees Demetrius following Hermia?
When Oberon, the King of the Fairies, spots Demetrius following Hermia, he realizes that something has gone terribly wrong. Oberon knows that Demetrius has persistently rejected Helena, even as Helena has continued to relentlessly throw herself at Demetrius's feet and begged for his love. Demetrius claims to love Hermia, who is already in love with a different man—Lysander.
Thus, Oberon realizes that Puck has made some sort of serious error somewhere along the way: Demetrius was intended to receive the potion so that he would fall in love with Helena. Oberon sends Puck to go fetch Helena, and Oberon insists that he will place the potion in Demetrius's eyes himself. As a result of this magic, Demetrius will then fall in love with Helena. This would ideally fix the love triangle situation that has plagued the four lovers.
In the play 'A Midsummer Night's Dream' by William Shakespeare, the author sets up a number of 'deliberate mistakes.' Deliberate on the part of the author that is - the mistaken eyes are a genuine mistake made by Puck. When they see Demtrius following Hermia, they realize they have gone wrong somewhere. Demetrius comes with Hermia still pleading for love. He falls asleep after she has gone. Then Oberon tries to put things right and undo the damage caused by Puck. He puts the magic juice on to the eyes of the sleeping Demetrius, then sends Puck off to find Helena and bring her back. So Demetrius then declares his love to her instead, and she does not think this is funny, thinking the only reason can be that they are playing a joke on her and making fun.
The short answer to this is that he realizes that Puck has put the love potion the eyes of the wrong man.
Puck was supposed to put the potion in Lysander's eyes. But instead, he put it in Demetrius's eyes. That is why Demetrius is following Hermia.
To correct the miskate, Oberon tells Puck to go and find Helena. Puck is supposed to bring her back. Oberson says that he himself will put more potion in Demetrius's eyes. This will make Demetrius fall in love with Helena and all will be well.
Of course, it doesn't exactly happen that way...