If you're slicing a design from a designer in Sketch, you may find some horizontal/vertical red bars. Here's a quick little how-to on how to get rid of them.

1. Go to the Sketch artboard with the red bars
2. Go to View in the top Sketch menu, in the dropdown, hover over Canvas and then Layout Settings.
3. You'll see something like this:
   
4. Uncheck Columns and Rows, then press Confirm and hey presto, bars are gone!