First things first, I'm kidding, we all know that 100% data accuracy is the elusive white whale in the Salesforce ocean. May we all be known as Ahab when we retire.
When I first started with Process Builder, this error drove me crazy. I have no idea where my process broke, based on this error message alone.
In protest, I would delete the entire process (that'll show 'em!), and build what I needed via Formula Fields and standard Workflow rules. However, I spent more time each week on data clean up than it took me to build the process initially. Embarrassing…
If I had a dollar for every person who asked about Field Sets, I could buy a If I had a dollar for every person who asked about Field Sets, I could buy a remote controlled tarantula..