Client Concentration
The share of a network's nodes running the same software implementation, where a single bug could affect all of them together.
Independent teams write separate programs that implement the same protocol rules, so that a bug in one is caught by disagreement with the others rather than being accepted as truth. When one implementation runs on a supermajority of nodes, that safety property inverts: a bug in the dominant client can be finalized by the network as though it were correct, and a crash in it can stop block production. Ethereum tracks execution-client and consensus-client shares publicly for exactly this reason, and the thresholds of concern mirror the stake thresholds, around one third and two thirds. Client bugs have in practice caused temporary chain splits on major networks, resolved when operators running the affected software patched and resynchronized.
In practice
If a client holding more than two thirds of nodes wrongly accepts an invalid block, that block can be finalized, whereas the same bug in a minority client causes only those nodes to fall out of consensus.
The common misunderstanding
That running more nodes fixes client concentration, when the risk depends on which software those nodes run rather than on how many of them there are.