Freed-Witten anomaly is a global ambiguity of the string path integral measure in the presence of D-branes. It turns out it uniquely determines, by imposing its cancellation, the topological type of both the A and the B fields when restricted to the brane, as it naturally involves the interplay between open and closed string degrees of freedom. After introducing the suitable mathematical framework provided by the theory of gerbes with connections, I will try to characterize the nature of the gauge bundle on a brane in the most general closed string background. Furthermore, I will go over a case by case analysis, focusing on how it can account for fractional RR and Page charges and showing the physical relevance of the resulting K-theoretical improvement for classifying D-brane charges. Reference: L. Bonora, F. Ferrari Ruffino, R. S., arXiv: 0810.4291