What happens if i accidently hit a vein while giving myself my insulin injection for diabetes?

No big deal: This is actually not uncommon and it could happen to anyone. The simple answer is nothing major will happen to you if you do this. You most likely will have a small bruise. The Insulin also could "peak" quicker than before so it might be prudent to check your blood sugar more on the day that this happen. We give Insulin through the iv's quite frequently to treat hyperglycemia in the hospital.

Probably nothing: If you inject Insulin and hit a vein most likely nothing will happen other then causing the vein to bleed resulting in a small hematoma. If you should inject into a larger vein then you could potentially cause your blood sugar to drop from the insulin. This result is very unlikely since Insulin injections are not done in areas of big veins.