How much can a sling take before it yields, breaks and eventually drops the load?
No one really knows until it happens.
Inspect your slings, use them correctly and most of all do not overload them to failure.
Picture of a sling and hook a customer where things happened.
CREWS CRANE TRAINING INTERNATIONAL, INC. 
I’m not a fan of using wire or chains to max load just because of this problem.
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Yes I agree when a hook, sling or rigging hardware is used passed 70 percent or used incorrectly this can happen. Most times it is operator and not equipment as the fault.
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