The rotating "anti-pick" plates get out of alignment if you turn the key while taking the key out, or turn the key while inserting it.
Just use the tip of your key to realign the first plate with the second plate, and the second plate with the third plate, until you reach the bottom of the key way. Then turn the key, and the lock opens.
The rotating plates seemed very loose on 2005 model flat-key locks. The flat-key locks currently reaching bike shops use the exact same design, but the rotation plates don't rotate easily. The only way to get them out of alignment is to insert the key and begin turning the key before the tip of the key hits the bottom of the key way.