I bought two "lifetime guarantee" umbrellas for my daughters at Amazon back in October 2016: one pink, one purple.
By February, one of the struts on the pink one had broken, and the vendor replaced it. Today, I can report that one of the struts on the purple one has broken.
I could have chalked up the first break to misuse or a manufacturing flaw. Two umbrellas, with identical breaks? They shouldn't really have slapped that "lifetime guarantee" on these things.