Today, I found 150 really awesome free vector icons that were in AI format. The problem was, they were in one file on the same layer! The icons were laid out in a row and column pattern, but I had to create guides manually. Once I created the guides, I proceeded to use the handy Object > Slice > Create from Guides feature to chop each icon into it’s own file. After a very long time processing, and much to my dismay, the slices generated were not cut neatly along the guides! There were a bunch of erroneous slices that corresponded in no way to my nice little guide array. It turns out some other people have had this problem and from the several things I read and was able to piece together, there’s a workaround (or at least a series of things I did that lead to a working solution).

1. I set my ruler units to Points.
2. I made sure View > Snap to Point was checked.
3. Then I turned on the Grid (Ctrl + Double Quote) and dragged out my guides so they snapped to the grid.
4. I grabbed the Select Slices tool and pressed Ctrl + A to select all my slices.
5. Following that, I ran  the Object > Slices > Create from Guides command again.
6. This time, in about 10 seconds, I was presented with a beautifully sliced AI file with 150 pristinely cut slices.
7. From that point, I ran my Save for Web and Devices with Images Only like I have a million times in the past and ended up with a folder full of glorious png icons.

From what I can gather, this issue is common if you use a metric other that Points. Illustrator displays at 72 points per inch (ppi), which must mean their is a bug in some unit transformation somewhere in the program. Let’s hope Adobe gets that fixed so that creating slices from guides becomes a unit agnostic operation.