(Coming From Experience)
Any vegging plant that has multiple things stressing it out at once will cause it to flower in veg...I did it twice last year due to pH fluctuation, supper cropping, nitrogen toxicity, root bound, 20 degree temp and humidity fluctuation. If all of these conditions mentioned at the same time effect a vegging plant they will cause veg growth to halt. u don't get any new shoots out of the tops for two weeks... (If u don't do any treatment and leave it in veg it will take 2-3 weeks to revert back to veg growth as if u were to throw it in flower and back in veg u would have a growth halted stressed plant) Also fuzzy calyx pistils turn brown and u get a bud like top at all top nodes and side growth continues veg until effected . done it with 2 different strains thought they were auto-flower but they weren't because the clones i had (cut before the stressed flower production)grew out to be vigorous vegging plants for over 2 months, i also forgot to mention that both plants that had this stress induced flowering were plenty mature over 2 months old with sexy white pistils at every node before they started looking like 2-3 weeks in and i got worried, so i did anybody would do if they seen bud on there plant, i put it into flower and the crystals kicked in over one night, as for my other plant that had the same condition which was smaller in size so i thought it wouldn't produce much if flowers as is.....i decided to experiment.....LST'D every main branch and let all undergrowth grow up for at least 3 days to a week, still no vegetation from the tops waited until every premature chute was 2 nodes old, and cut every main branch (6 branches over 7" each) off after the premature chutes and that's all i was left with was 6 premature chutes about 2 inches high that quickly grew into a bush. The only problem i contemplated would be the spread of root rot so i decided to clone the plant and disposed. Since then i veg'd and harvested all the clones i had previously cut off the stress revived plant and did not once see the same conditions in that cycle. stress can make your plants think they are going to die and then they start to protect themselves, i had sugar coated crystals on a vegging plant on 18/6 and it looked 3 weeks in except for the brown pistils all over indicating stress...ROOT BOUND WILL CAUSE FLOWERS IN VEG NOT PRE-FLOWERS...IT WILL TAKE TRANSPLANT AND 2-3 WEEKS OF VEG AND NO STRESS TO REVERT! mature plants are way more sensitive if it happens cut any branch off with flowers leave the vegging growth and cater to her, or if shes big enough stick her in and watch her grow quick and dense