There is a solution to this problem. What you need to do, as I did with my IKEA bed when I finally figured out what was going on, is take the mattress off and fix the tension on the thin adjustable metal strips that attach the center bar to the sides of the frame so that they are as tight as possible! I did this by pushing my body weight against the side of the bed while fastening the screws. Once these are tight enough, the sides of the bed won't buckle and the slats won't fall through anymore. Naturally, this is NOT made clear in the IKEA instructions, but it should save you the trouble of returning it.
