Aside from the obvious bad rigging (you did see that, didn't you?), plastic buckets have a serious weak point: The metal bail that forms the carrying handle. This is nothing but a piece of bent steel wire that hooks into the plastic rim of the bucket. Given sufficient (over)loading, the plastic bucket can deform and let-go of the bail (literally tearing the plastic), and/or the metal bail can bend and lose it's grip on the bucket. Either way - this thing is going down.
Safe lifting of materials is usually best accomplished when the lifting system encompasses the load, this places the load on the lifting device and not on the object being lifted.
These slings are rated for 200 Pounds and are inexpensive (relative to the cost of a new noggin'). The handle is constructed of reinforced Nylon, and has an extra layer across the hoist point. It's pretty snazzy, too!
Don't be a doofus and drop your load - secure it!
More info at: www.thecircuscompany.com
Fly the friendly skys . . .