Sorry for bringing this back but....
The other day I was looking at my shelving i bought from the depot and I had turned a 6 ft. shelf into two 3 ft racks placing my carboys all on the bottom and the upper shelf was filled gallons and a few magnums etc. along with misc stuff.
I noticed that the sheeting that came with it is starting to bow a little on the top shelf. The bottom shelf has the support of 2x4's so no worry there.
Over time moisture has started to work at weakening this and eventually disaster would strike me. Now the only thing on these top shelves is a few empty bottles, paperwork and misc light stuff.
Everyone please from time to time look over your set up and see if pending doom is heading your way. One bottle/carboy, even a gallon falling can take out a few larger carboys below.