I was just munching through a loaf of bread, the use by date of which was yesterday. I just noticed that my uneaten slice (I’ve already eaten five) had a little green mould in the corner. Assuming that there was a little on at least one of the slices I’ve already eaten, is there any risk to my health?
The risk isn’t zero, but it’s darn close.
It’s more much dangerous if you eat the moldy bread intentionally.
Actually, some common species of mold make what are known as aflatoxins, which are among the most potent carcinogens known to science. On the one hand, it’s probably not all that dangerous. On the other hand, the data’s not great, since it’s just about impossible to link a case of cancer to a slice of moldy bread.
Just something to keep you up nights. 
Remember that TV show Sliders? In one episode John Rhys-Davis concocts penicillin using moldy bread. He warned that he hadn’t been able to refine it because of the lack of proper scientific equipment and that it could be fatal. So…
What are the dangers of roll-your-own penicillin? Suppose you grow a bunch of bread mold in a medium and drink it? Will it kill, or cure? (Assuming, of course, that you’re not allergic to it.) Was this scene based in fact? Or was it just a plot device?
That asked, I’ve accidentally eaten small amounts of bread mold with no ill effects, but I wouldn’t recommend it since I have no idea what large amounts would do.
A favorite ‘experiment’ of one of my mother’s high school science teachers was to leave slices of bread in a window sill for much of a semester, wait for a thick growth of mold to appear, shake off the dust, and eat it. So, what, about seven slices of moldy bread a day, twice a year.
He retired - thankfully - before I got to HS, but the guy is still alive, active in my community, and pushing 90.
See!
See!
He got moldy!