Hi there!
For our two largest atlases, clicking on them in the Project window (one after the other, or even one, then several other atlases) will cause Unity to crash. This also happens when both Atlases are used in the same scene - clicking play to run the scene will cause Unity to crash. We were using version 2.3.0 but recently updated to 2.5.6 in hopes of alleviating the issue, but no dice.
Examining the atlases produces this error:

Playing the level produces this error:

Here are the Settings for each Atlas. I didn't embed because the images are rather tall:
https://dl.dropboxusercontent.com/u/2954274/Capture3.PNGhttps://dl.dropboxusercontent.com/u/2954274/Capture4.PNGWe've been working around it, but it is proving to be especially annoying for our project's artist. Do we need to reduce the size of the atlases?