Case: the AFCI that would not stop tripping
In one paragraph
Two electricians before her had replaced the bedroom AFCI breaker, twice, and the third one tripped within a week like the others. Marisol Tran runs the method instead of the parts counter: verifies, isolates, tests dead, and finds two 1978 bedroom circuits sharing one neutral in a ceiling box, wired by a previous owner, so that the AFCI sees the other circuit's return current as a fault and trips exactly as designed. The fix is a wiring repair, the code makes it a two-pole breaker or a handle tie with grouped conductors, and the whole call fits inside a $750 service and repair contract. The names are invented; the fault is the most common one there is.
The rule
An AFCI breaker trips on a shared or crossed neutral because the return current of another circuit is, to the device, a ground or arc fault; the repair is to separate the neutrals or, where two circuits properly share a neutral as a multiwire branch circuit, to supply them from a two-pole breaker or single-pole breakers with an identified handle tie, group the conductors at the panel, and pigtail the neutral at every device (2025 CEC 210.4(B), (D); 300.13(B)). AFCI protection itself is required on the circuit by 210.12(B), and replacing the breaker is not the repair.
Why it matters
The AFCI service call is the one that separates a parts-changer from an electrician, and the customer has usually paid for two parts already. Finding the shared neutral takes forty minutes with a meter and a lockout; not finding it costs the customer another breaker and costs you the customer.