Best way is to understand how weed flowers...during lights-out, the flowering hormone builds up. When the lights are on, the flowering hormone is destroyed. Changing from 18/6 to 12/12 alters the balance of the hormone creation-destruction cycle, increasing the level of flowering hormones, causing the plant to flower once a flowering threshold is crossed. (which varies by plant)
During mid-flowering, the levels of flowering hormone reserves are high enough that one fuck up on the lights is less likely to mean it falls back to non-flowering levels during an extended lights-on period.
If you're early in flower, the flowering hormone will not be so abundant, so lights timing issues can be more problematic.