1

Just as you reach the top, you lose your grip and fall.

2

You fall and take anyone climbing with you down as well. Alternatively, you fall and land on someone below you, both taking half damage from the impact. In either case, the other person can roll to dodge out of the way.

3

You get stuck halfway up and can't manage to get back down. Someone else can come help you up or down, or you can just let yourself fall.

4

Halfway up, part of the wall crumbles away (or a branch snaps, etc). You fall and are pelted with falling debris for good measure. The wall (or whatever you were climbing) is too damaged to try to climb again.

5

Your climbing equipment breaks. You can make it the rest of the way with another successful climbing roll (without any bonuses from the equipment). If you are not using climbing equipment, a random item you are carrying falls and breaks instead.

6

You disturb a nest of hornets, birds, scorpions, etc. They continue attacking you each round while you are climbing and for 1d4 rounds afterward (the tiny creatures can only inflict a maximum of 1 damage per round).

7

You reach the top... and then fall down the opposite side of the wall. If there is no opposite side (ie, climbing up to a ledge), then you just fall down the same side.

8

The climb was harder than it looked. You made it, but it took you twice as long and left you fatigued.