Bicycle Chain Slipping - Worn Chain and/or Crankset Teeth

The chain is slipping because either the chain itself is worn and elongated, or the teeth on the crankset are worn down, preventing a proper engagement between the chain and the gears. This is often characterized by the chain 'riding up' on the teeth before slipping off.

Examine the chain for excessive stretching using a chain checker tool. Inspect the crankset teeth for a 'shark fin' shape – a sign of significant wear.