[edited to reflect an issue in my sample]
This refers to creating an advanced function or cmdlet with different parameter sets where the parameters can be passed in from the pipeline by property name.
If you pipe an object with properties that match both sets, parameters are bound using __AllParameterSets, the AmbiguousParameterSet error is processed, but the end/endprocessing block still executes.
If you specify parameters manually, the command completely fails, without executing the end/endprocessing blocks.
This happens in both PowerShell v2 as well as PowerShell v3.