Are asynchronous CPM "processes" queued ever removed due to errors in processing? 

Environment:

Process Designer, Custom Process Model (CPM) / Service Process Model (SPM)
All product versions

Resolution:

When an asynchronous CPM (that is triggered through a Process Designer mapping only) is triggered the system schedules the event handler to execute using a queuing system. If the first attempt fails for some reason (due to an uncaught code exception or otherwise), the event handler execution will be retried automatically up to nine times. The initial 3 retries are attempted with 3 minute delays in between. And the next 3 retries are attempted with 20 minute delays. Then 3 more retries with 3 hours, 12 hours and 24 hours respectively.

The inherent retry For more information, refer to the following: 

Answer ID 6604: There are two types of execution for Object Event Handlers