Update using non-existent fields does not throw an error

Greetings!

An update query is apparently succeeding, even though the query refers to fields that do not exist. Here's the query:

update inventory set
x_coordinate = (select x_coordinate from bases where base = '101'),
y_coordinate = (select y_coordinate from bases where base = '101')
where charge = 100

-- select x_coordinate, y_coordinate from bases where base = '101'

When I run the update query, it tells me that the query succeeded and that four records were updated, which is what I expect. But when I looked at the inventory table, I found that the four records were unchanged. So, I tried to check the values of the base coordinates by running the select statement shown above. That statement threw an error complaining that x_coordinate and y_coordinate did not exist. This is correct; I should have been querying a view that includes those fields. But why didn't the update statement throw an error?

RobR