Many transition-metal complexes MLn decompose diazo compounds N-2=(CRR2)-R-1 generating metal-carbenes LnM=(CRR2)-R-1 which transfer the carbene group to other substrates, constituting an important tool in organic synthesis. All previous reports have shown that the (CRR2)-R-1 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 (CRR2)-R-1 ligand has undergone a modification from the initial diazo reagent. When Tp(Ms)Cu(THF) (Tp(Ms) = hydrotris(3-mesityl)pyrazolylborate ligand) was reacted with N,N-diethyl diazoacetamide [N-2=C(H)(CONEt2)], the stable copper carbene Tp(Ms)Cu=C(H)(NEt2) was isolated, resulting from a decarbonylation process, with carbon monoxide being trapped as Tp(Ms)Cu(CO). The simultaneous observation of products derived from the intramolecular carbene insertion reaction into C-H bonds demonstrates that the expected Tp(Ms)Cu=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.