In general, you can’t hurt yourself by taking Vitamin B12.
In specific, however, you should remember that B12 is used as a treatment for anemia, and that a lack of B12 can cause anemia. Pernicious anemia is caused by your gut not being able to process food-borne B12, which is why folks with pernicious anemia get B12 shots instead of taking it orally.
B12 helps your body make blood.
So as the Mayo Clinic points out, if you take a lot of B12 and your body increases your blood volume by making more blood (a good thing), you can actually end up with too much blood. And that would cause a sort of high blood pressure, which is signalled by redness in the face, etc. (And if you already have high blood pressure, THAT WOULD BE BAD.)
Now, obviously you don’t want to tempt your doctor to bring out the leeches or the bloodletting equipment! (Although I guess you’re okay if you live in a paranormal romance and have a vampire around the house.)
Therefore… if you have really high amounts of micrograms listed on the side of your Vitamin B12 bottle (like 5000 mcg), do not pop them like candy. If your face looks weirdly red, ease off. And if you already have high blood pressure or heart problems, talk to your doctor about safe amounts of vitamins, as customized for you.