Are there any side effects or precautions regarding fillers?

The filler seems to be effective, but I'm a little worried after reading about the side effects.
I'm worried because I've read that if fillers are injected incorrectly, they can clump or result in an odd shape.
Still, since fillers are a common procedure and there are many claims that they are safe, I am even more confused.
I'm wondering if there are any precautions or things I absolutely need to know or check before getting fillers.
It would be helpful if those of you who have experience with fillers could share the pros and cons or things to be careful about.