It all depends on how theyre stored and bottled. Ive used supps in pill form that had years of shelf life left, yet due to improper storage which could be down to ambient temperatures, humidity, sunlight etc, either visibly went bad (solidified or turned into strange colours) or lost potency. On the other hand im currently using some supps which were supposed to have expired 2 years ago, yet they work great still.
Also keep in mind that some are labeled with expiry dates and others with "best before" d ates, which means that technically they can still be used. Of course as the other poster said, the worst that could happen is thst they lose potency. But then again, i do not trust expiry dates either. I think not even some of the companies know when their stuff expires and they just guesstimate a date...