Chain shuttling copolymerization (CSP) is a modern polymerization technique that allows in a one-step procedure access to multiblock microstructures of statistical copolymer blocks of different compositions, providing notably new thermoplastic elastomers. The process involves the shuttling of a growing copolymeric chain between two catalysts showing different reactivity ratios versus the comonomers. The complexity of the active species formed in the presence of two catalysts and a chain transfer agent can give rise to unexpected reactivities. They are presented and discussed in this contribution, including their possible origin when provided and the consequences regarding the design of catalytic systems for CSP.