According to the Docs for the standard
PoNumber is a valid field on the object. I have a query that references that field in an Apex class, and the class compiles, and the unit tests pass. All set.
But. Trying to run a few lines of code including that query in the Dev Console's Anonymous Apex window gives the error
No such column 'PoNumber' on entity 'Order'
I then tried running the query in the Console's query tab; same issue. Then I tried inspecting the object definition in the Console (Ctrl-Shift-O, Order.obj); that field isn't listed. But it's list in Setup, and I can reference it from code. Interestingly, I can edit/save the Apex Class containing the query from Dev Console. Here's a simplified version of my query which exhibits this behavior:
select id, OrderNumber, EffectiveDate, PoNumber from Order
Is this a known issue? Am I overlooking something special about Order.PoNumber?