13C isotopically non-stationary metabolic flux analysis (13C-INST-MFA) is an emerging technique for estimations of metabolic fluxes and pool sizes. Within the plant sciences, two studies utilizing this technique to characterize carbon metabolism have been published so far. Here, I examine these studies carefully. Readers unfamiliar with 13C-INST-MFA will obtain a critical understanding of the method and its findings. Readers working with 13C-INST-MFA are recommended to enter a phase of model validation to devise clear-cut protocols enabling robust estimations of specific fluxes.