Can accidentally eating maggots be harmful?

Maggots are the larvae of certain insects, in particular flies. Some kinds of maggots are used medically to debride wounds; one use of this is to thoroughly clean out MRSA infections as the maggots eat dead flesh but avoid living tissue. Other kinds of maggots can be parasitic, though, and those are probably the ones you're talking about; they're dangerous because, well, "parasitic" pretty much says it. Also, since maggots are fly larvae and flies frequently carry disease, maggots can be a source of infection as well (the medical maggots are specifically disinfected to avoid this).