I was testing some effects given to the player when they are on ice. When I climbed a ladder with these effects, it shot me into the air.
I have these effects running on a loop:
/execute at @p if block ~ ~-1 ~ ice run effect give @p minecraft:wither 1 50
/execute at @p if block ~ ~-1 ~ ice run effect give @p minecraft:jump_boost 1 200
/execute at @p if block ~ ~-1 ~ ice run effect give @p minecraft:slowness 1 4
/execute at @p if block ~ ~-1 ~ ice run effect give @p minecraft:poison 2 50
/execute at @p if block ~ ~-1 ~ ice run effect give @p minecraft:hunger 1 255
When you climb a ladder that touches the ground, you get shot into the air and will die because of fall damage
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I don't see the issue here. You had jump boost and hit the space bar when you were on top of the ladder to get on the slab.

In the video, I did not press the spacebar to jump onto the slab as you automatically move onto the slab if you move off of the ladder. The jump boost effect with an amplifier of 200 stops the user from jumping, but because you move onto the slab from the ladder, the game thinks you have jumped, which results in you being shot up into the air.
Enchantments and effects over max default value are not supported. See MC-10755.