For a long time now, I've argued that Lookup Fields in tables are One Bad Idea. I've even Blogged about it.
Recently it occurred to me that maybe it's not enough to TELL people Lookups are almost always bad. There's a world famous page that offers eight significant problems with Lookups. My complaints are actually focused on the fallout from Number 6 on that list. Lookup fields in tables simply make it far too easy for inexperienced users to screw up royally.