Booking on price alone and auditing nothing
The cheapest quote often prices out exactly the invisible steps that matter: autoclaved tools, full cure cycles, a chelating wash before keratin. Compare like-for-like scope across a few vendors and spend one minute inspecting the kit on arrival. The infection or ruined treatment you avoid is worth far more than the difference between quotes.
Skipping the patch test because you have coloured your hair for years
Sensitisation to hair dye builds with repeated exposure, so a clean history is not immunity. New vendor, new brand, or a long gap since your last colour all justify the 48-hour test. It costs two short visits or a self-applied dab; a facial reaction costs an emergency room.
Treating bridal bookings like a normal appointment
One artist cannot finish a bride and four relatives in a morning, and no trial means the first attempt at your wedding look happens on your wedding day. Confirm team size in writing, hold a photographed trial, and build an hour of buffer before photos.