If you accidentally eat one before taking out the 'poo pipe' can it make you sick? :0/

No you'll be fine

No; the matter in the "vein" in shrimp isn't harmful to us (unless the shrimp is otherwise contaminated or spoiled to begin with).

I've never heard of anyone getting sick from eating a shrimp that hadn't been deveined. Now if the shrimp had gone bad, that's one thing, but just eating one that still had the black vein, no. There's not that much waste matter in it to cause a problem. It can affect the taste slightly, however, and aesthetically, most people prefer to have the shrimp deveined before eating it.