When space becomes an issue,there's nothing else you can really do but stack your boxes.I get nervous going over 5 high for human or pet animal safety reasons.I also worry about the stress weight can put on the lower boxes in a stack even at 5 high.Should weight pose a problem or is the plastic really sturdy enough?How high have you guys stacked boxes either presently or in the past?Have you had mishaps doing this or found damage due to excessive weight on the lower units?I'm looking for stories,opinions,and evidence from you guys if you have any.