<p>I accidentally coded <code>SELECT $FOO..</code> and got the error "Invalid pseudocolumn "$FOO".</p>

<p>I can't find any documentation for them.   Is it something I should know?</p>

<p>Edit: this is a MS SQL Server specific question.</p>

A simple Google search brings up
this
from Oracle's reference:
A pseudocolumn behaves like a table
  column, but is not actually stored in
  the table. You can select from
  pseudocolumns, but you cannot insert,
  update, or delete their values.
I think that the error you got is simply because there is no column $FOO, so the query parser tests to see if there's a psuedocolumn named $FOO as a fallback. And since there is no pseudocolumn named "$FOO" (and there are no other fallback) you get the error "Invalid pseudocolumn $FOO". This is a guess, though. I'm no expert when it comes to databases.