I had to do IP injections on mice when I was working at a research laboratory. A mouse IP injection is not the easiest procedure, especially because mice are so small. There were times where I caused a hematoma in the mice's abdomen because the needle would accidentally puncture a vein.
Bleeding was a problem, not just because it would affect the research results but because it could be detrimental to the mouse as well. Even a small amount of blood could cost a mice its life, so that was always a worry. Thankfully, I got better at it with experience and had no problems 99% of the time.