Is it bad to have had a bit of foundation on the face while having laser hair removal treatment?

you want the skin completely clean when you get any laser treatments to avoid burn or lack of efficacy.  Best, Dr. Emer.

During laser hair removal treatment, any foundation with tint or color can lead to superficial burns. The skin should be cleansed and free of any makeup. 

I would advise against having anything on your skin during laser assisted hair removal. If your foundation has any color to it then it is possible that the laser could interact with the foundation and not with the hair follicles (since pigment is the ultimate target of the laser). If this interaction occurs then superficial burns may result. 

In general, I remove anything on the skin prior to laser treatments. The makeup itself won't cause a problem but it can interfere with the laser light, especially if the makeup is dark. As a rule, it's better not to wear makeup on an area you are going to have treated with a laser.

No, having the foundation on will most likely not impact your results. If however, your makeup clogs the pores and/or blocks the follicle to which the laser is to be targeting, it can prevent the successful removal by the laser. You should wait 4-6 weeks for the hair to go through all stages before having another treatment. Next time you have the laser, a fresh cleaned face is best.Thank you for your question and good luck!Melinda Lacerna, MD FACS