Sometimes, the images on button elements will appear like this. Seems to happen on certain images, but the images do NOT appear like that in the actual image file.

Has occurred on builds 1-9

See the top 2 buttons on the image provided.

Create a simple form modal with a lot of buttons, try with lots of different images.
The images specifically being used there can be found here: http://minecraft-ids.grahamedgecombe.com/items.zip