Possible scenarios;
you have transitioned from a mainLocation to a mapLocation, and your loading dialog does not cover the entire area of the screen, and/or you are not using a fade transition....if this is the case either use a fade effect or ensure that the loading dialog covers the entire area of the screen.
you have gaps inbetween your tiles, that allow the voided area(black area) to eek through, if you are using png graphics with soft transparencies you'll notice the jaggies appear on all visible images, however if you are using pict graphics it would be less subtle...this coupled with the first anomaly could possibly create your bug.
i'll have to rattle off some snapshots later today.
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(*)Cute Girls work at Target.