I somehow managed to play a blank tile as a blank (6th row, 13th col):
and got the double word score for TACT.
Is this a glitch, or a legitimate move? The HTML for this tile (note that zynga numbers rows starting with 0):
<div class="space_13_5 space dw has_tooltip mt recent"><span class="tile lette\ r-" data-letter-id="0"></span>
(It's game number 3950502083 if anyone from zynga's reading this)