Developers do not properly take into account possible problems on the Internet. This fact can cause problems in the operation of blockchain systems. The subject of our analysis was Ethereum due to its short block creation time and, as a result, a strong reaction to faults in the network. We checked the possible threats from the miners during the unreliable work of the Internet. For the study, we conducted many simulations of the work of Ethereum with different delays in the propagation of blocks and various mining power allocations. The results showed that if the duration of the troubles is more than 10 h, a miner with 26% can capture the entire Ethereum network, but this requires the intervention of people controlling other nodes. This is possible because it will be more cost-effective for them.