Mainly failure, recurrence, and hyperpigmentation

The worst things that could potentially happen from treating spider veins would probably be failure of treatment, recurrence of the spider veins, and especially hyperpigmentation.

Hyperpigmentation happens when the tiny thin walled veins burst from the treatment and blood leaks into the tissues causing a sizable bruise. As the body absorbs the bruise, the iron particles in the hemoglobin molecules linger in the tissues for months causing a brown stain. This usually resolves but very slowly.