The stale block is a phenomenon that occurs every now and then in a cryptocurrency network. It's not necessarily a security concern, but it can affect the network in a few ways. The first effect is that it indicates poor network propagation, which results in wasted work. The second effect is that the network will have lower proof-of-work, which makes it cheaper to attack.
Bitcoin is not immune to this problem. On January 27, 2020, a block turned stale, causing money on the Bitcoin blockchain network to double-spend. As a result, Bitcoin is now considered a stale currency. Many observers have been referring to Bitcoin as a "true shitcoin".
