I'll try copying the db and moving to another instance, but I'll have to do that later as I'm on a deadline. I'm just going to add the extra...

CHECKDB found 0 allocation errors and 0 consistency errors in database 'testdb'.

<-- the real sad face

[1] Database testdb: Check Data and...

Tried rebooting. Not reinstalling. I'm running checks in a second. Thanks for the help guys

Thanks for the reply! Unfortunately all queries show the same plan:

|--Clustered Index Scan(OBJECT[testdb].[dbo].[invoice].[PK_invoice]))

(edit: should be |--Clustered Index Scan(OBJECT: ([testdb].[dbo].[invoice].[PK_invoice]))

After further testing, if I ADD a column to the select list it returns perfectly. It seems to be the unique combination of columns listed.