The Complete Guide to English Spelling Rules

A censer is for burning incense, but a censor is a person.
A quitter is a person who quits, but quittor is a horse disease.
Lumber refers to wood, but lumbar refers to the spine.
Note that finger is an anomaly because the er is part of the root word. It also breaks the hard g rule.
Some dictionaries offer adviser or advisor, also conjurer or conjuror. As these persons are carrying
out an action, the ending should be er.