As far as I know, quitting smoking cannot cause a false positive pregnancy test. Here are some things that can:
soap/ detergent (This is why women should urinate directly on the stick rather than into a cup that may have soap or detergent residue.)
HCG fertility shots
anticonvulsant medications
tranquilizers
diuretics
Parkinsons medications
trophoblastic tumors in the reproductive system
hysterical pregnancy