It happens occasionally, but not often enough for me to be concerned about, it's one of the recommended spare parts to keep on hand, it's mentioned in the FAQs sticky. FWIW, I've had over fifty H&Rs, all modern made 1987 and later, have only had one transfer bar break. I've had one get floppy on me, and I replaced it before it broke, it's pretty easy to spot one that's ready to come apart, they don't usually actually break, the two pieces separate, the connecting pin can be peened to tighten it back up if needed, Numrich carries replacements.
Tim