Two incidents bracket Cook's career and are usually read as pointing in opposite directions from the Crookes sittings. In 1873, before Crookes's investigation began, the collector William Volckman seized the materialised Katie King mid-seance and claimed the figure was Cook herself; the sitting ended in confusion, Cook's supporters accused Volckman of assault, and no independent examination settled the question either way. Crookes began his own sittings not long afterward, and his eventual endorsement is sometimes read as a direct rebuttal of the Volckman incident, an attempt to test properly what Volckman's grab had only asserted.
The more damaging incident came in 1880, well after Cook's reputation had moved on from the Katie King sittings to further materialisation work under her married name, Florence Corner. At a sitting hosted by Sir Charles Blandford, the young writer George Sitwell seized the materialised spirit and found, by his account and that of other witnesses present, Florence Cook herself, wearing only a chemise over her ordinary clothing where the drapery should have been. Unlike the Volckman incident, this exposure was witnessed by multiple observers in circumstances contemporaries treated as considerably harder to explain away, and it effectively ended her standing as a credible physical medium even among sympathetic spiritualist circles.
Historians who have revisited the case note that Cook continued to attract sitters afterward, which some read as evidence of a devoted following unmoved by exposure and others read as evidence of how little a single damning incident could dislodge belief already invested in a medium's gifts.