Many transition-metal complexes ML_n decompose diazo compounds N₂═CR¹R² generating metal-carbenes L_nM═CR¹R² which transfer the carbene group to other substrates, constituting an important tool in organic synthesis. All previous reports have shown that the CR¹R² fragment at the metal-carbene remains intact from the parent diazo compound. Herein we report the detection and isolation of a monosubstituted copper carbene where the CR¹R² ligand has undergone a modification from the initial diazo reagent. When Tp^MsCu(THF) (Tp^Ms = hydrotris(3-mesityl)pyrazolylborate ligand) was reacted with N,N-diethyl diazoacetamide [N₂═C(H)(CONEt₂)], the stable copper carbene Tp^MsCu═C(H)(NEt₂) was isolated, resulting from a decarbonylation process, with carbon monoxide being trapped as Tp^MsCu(CO). The simultaneous observation of products derived from the intramolecular carbene insertion reaction into C–H bonds demonstrates that the expected Tp^MsCu═C(H)(CONEt₂) complex is also formed. Experimental data, DFT calculations, and microkinetic models allow us to propose that the latter undergoes CO loss en route to the former.