If you scroll down you can see that some people drew completely unrelated things. One guy doodled a stick figure. Another wrote "$0.01!! REALLY?!" The last guy did the drawing, then wrote "i hate u" over top.
That kind of stuff is pretty rare. Turkers are pretty hard-working/honest people on the whole I guess out of 10,000 maybe 10-20 screwed around. That's less than .2%.
Looking at the bill, the number of bad cells seems far higher than you suggest. Some people didn't absolutely screw around but many did not finish the work. The percentage gets pretty bad in some of the high detail areas.
I'm a huge fan of MT, and do think that it can be used to get pretty accurate/high quality results. You can actually have multiple people work on each HIT and then cross-check or chose the best
An algorithm you could use: try three times, prefer the most similar two ("map"), look in those for puzzle-piece matches with the nearest neighbors ("reduce").
Guess the Mechanical Turk isn't good for, say, important data entry (without a system to set up cross-checks).
Pretty cool, though.