If you are using Apex Invocable Actions in your Process Builder be careful. This Known Issue is more broad then the way it is written here. These can just disappear from your Process Builder and for more than just standard fields.

This was a Known Issue in the Spring release and the workaround was "dont use them". It was marked as fixed in the Summer but it is not. I have a Process that ran last night and errored wildly. Open this morning and I see there are no Actions in the steps. Each of these had Apex Invocable actions as steps and they were there on July 29 at 5:10PM.