Reliability is one of the key specifications of a system. For graphene synthesis by chemical vapor deposition (CVD), previous studies have shown that a trace amount of oxidants can significantly affect graphene growth, either positively or negatively. Other than deliberately introducing oxygen, oxidant impurities may be introduced by the air leaked into the system or from impurities of the reaction gases, which are normally constant and can be balanced by hydrogen and carbon precursors. Here we report a systematic investigation on the effects of Cu contamination on graphene growth, which is very common due to Cu evaporation at high temperature. The accumulated Cu deposited on the quartz tube are inevitably oxidized to form Cu₂O and CuO, which result in an introduction of water either due to the desorption of water adsorbed on the rough surface or the reduction of oxides by hydrogen and methane. In addition, the introduction of water in this way is not constant, leading to a non-steady state for graphene growth and may significantly reduce the system reliability. Our finding brings caution to the study on graphene synthesis and the development of its industrial production.