Most of us have done it at least once, you see the smallest spot of mold on the crust of a loaf of bread, and you figure that if you just cut that little part off then there'll be nothing to worry about. The problem here is that the mould we see on food is really just surface spores. Like any other plant, mould has roots and branches — and they travel deep.
Photo by Keenan.
By the time we see mould growth on food, it's already pretty heavily embedded (it's quite tiny, after all). The USDA has an entire page set up just for mould, and it has this to say about that growth:
When a food shows heavy mould growth, "root" threads have invaded it deeply. In dangerous moulds, poisonous substances are often contained in and around these threads. In some cases, toxins may have spread throughout the food.
Basically, it's bad. There are exceptions though — for three things. Salami, which is already technically covered in the stuff, can just be scrubbed really well if uglier spores show up on its surface. Hard cheeses can still be used if mould shows up, but the mouldy section has to be cut off a full inch deep into the product. The same goes for harder fruits or vegetables, if there's a spot, cut off a full inch. Anything else is tempting fate, or at least the mycotoxins.