You can't trust a ladder. It will always let you down.
Why this joke works
Let Down in Two Directions
To let someone down means to disappoint them, which makes the opening sound like a warning about an unreliable companion. A ladder can also bring a climber physically downward, giving the same phrase a literal job. Read the first sentence with serious suspicion. Keep the second sentence calm and certain, without emphasizing “down.” The straight delivery lets the emotional meaning arrive first and the ladder's vertical meaning follow a moment later.
A closer look
A Helpful Object Receives a Personal Betrayal
The opening warns against trusting an inanimate tool, inviting a story about instability or failure. “Let you down” normally describes disappointment, but descending is one of a ladder’s intended services. “Always” transforms reliable downward access into a permanent character flaw. The joke’s confident accusation works because a properly used ladder is not betraying anyone at all; it is doing exactly what was requested.
Fact behind the joke
Safe Ladder Use Depends on Inspection and Stable Contact
OSHA advises inspecting portable ladders before use, following markings, keeping rungs and feet free of slippery material, facing the ladder, and maintaining three points of contact while climbing. Damaged ladders should be removed from service. A ladder can provide controlled access both up and down, but placement, condition, load, electrical hazards, body position, and user behavior determine whether that access is safe.