Here's a fun one...sorry if this has been posted before or is a known error. I searched but couldn't find a thread on it.
If the column you are using to create your running total is too far down your list of fields, it will return 0's or incorrect values, even if you have a sort tool in place before it. The total number of fields doesn't seem to matter. It's the actual position of the field.
If the field is past the 256th position, the tool appears to loop back to the top field in your list and try to calculate the running total using that instead. So if your calculation field is the 260th field, the field that is ACTUALLY being totaled is the 4th field. If this is a string field, you'll get some weird conversion errors, but if it's numeric it will not show an error. In our case this returned values in the trillions, but it's absolutely possible that the numbers returned could look correct but be based on completely wrong data.
I've attached images showing the difference in behavior when the field selected to 'Create Running Total' is in the 256th position vs 257th. I might be using the wrong term but it seems like an integer overflow issue.