Typically it’s best to discover them before closing. But of course there are situations where you may not discover something till after the closing. As soon as something like that comes to your attention it’s important to contact an attorney to see what your legal rights are. In Florida the seller’s obligated to disclose all known defects that materially effect the value of the property. So if the seller was unaware of the mold or the lead-based paint then you probably don’t have any recourse against the seller.