Process instance does not exist

Certified Senior Developer

I discovered this issue because I found out a query into a task report wasn't returning some expected data. I took a look at the report itself, and the data was missing from there, so it wasn't a problem with the query. I went through the process models to see if I can at least pull out information about the specific process instance that wasn't showing up in my task report. I went into the parent process model and tried to open the subprocess instance, but found this error: 

How does this error show up? I suspect one way would be for a user to open up process instances on the subprocess level and delete some there, but I haven't deleted any recently and I don't think other people working on this project have either. Are there other ways for this error to show up?

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