At image 1, you will se the erro tiles (bad )! Image 2 is good tiles !
Image 1:
Image 2:
This glitch happend when you blend tiles near right and down area of your map!
If you dont understand what is say:
Go to map editor, map size is 50X 50
On the up-left of your map, creat tiles like in image 2 !
then move hole map > Move right and down, you will see it happend !