Box dye. Home colour is formulated to grip and stay. It often sits unevenly — darker at the ends where it has been applied repeatedly, and it can lift orange or muddy rather than clean.
Banding and stripes. Highlights placed in the same sections repeatedly, or foils packed too close, leave visible horizontal lines. The hair reads striped rather than blended.
Brassiness. Yellow or orange coming through blonde. Sometimes the lightening did not go far enough, sometimes the toner has faded, and sometimes the underlying pigment was never properly cleared. These need different solutions, and telling them apart is most of the work.
Blonde work that did not turn out as intended. The most common arrival. The tone may be off, the placement uneven, or the ends more compromised than expected.