In Harper Lee's To Kill a Mockingbird, in Chapter Six, what goes wrong with the children's escape plan?

In Harper Lee's To Kill a
Mockingbird, Chapter Six brings Jem, Scout and Dill to Dill's last night in
Maycomb before returning home. It becomes clear to Scout that the boys are hatching a
plan that they are not eager to share with her. They finally admit that they plan to go
to the Radley house and peek inside a window to try to get a look at Boo. Scout is
opposed to the idea, but Jem says if she doesn't like it she can shut up and leave.
(More and more Jem and Dill spend time together that does not include
Scout.)

Scout decides to go with them. Dill looks through a
window and sees nothing. Jem does the same thing, but a creaking step must give him away
for first there is a shadow that passes as Jem is on the porch; and when it seems the
"coast is clear," all three kids fly out of the Radley
yard.

Soon, however, they hear the sound of a shot gun
which scares the life out of them: as they run, Jem is nearly sobbing in fear. They
decide to climb under the fence that leads into the school yard. Jem holds it up for the
other two. Scout and Dill get through and go on until they realize that Jem is not with
them. Turning around, they find him caught on the fence. Jem kicks off his pants and
leaves them still attached, and in his shorts, runs with the other two to get safely
away.