I had this happen to me once in an old 24 person raid from 2.0 era. Like you I found myself without a shield, which was a real problem cause you needed one for tank stance back then. The answer I found was that I had upgraded my old shield to a higher ilevel one but never updated the gear set. Since tanks don't get instant queues for 24 person raids I did a bit of mining while I waited. When it popped I switched back and quickly accepted. Since a job only needs a Main Hand to work the game gave me no warnings other than the standard not all gear found massage, which I failed to notice in my haste to click the ready button.

Since then I've always made a note of updating my gear set the very moment I get anything new so it doesn't happen again.