https://en.bitcoin.it/wiki/BIP_0050A rule should be something agreed to every nodes.
However, according to BIP0050:
This would be an issue even if the entire network was running version 0.7.2. It is theoretically possible for one 0.7.2 node to create a block that others are unable to validate, or for 0.7.2 nodes to create block re-orgs that peers cannot validate, because the contents of each node's blkindex.dat database is not identical, and the number of locks required depends on the exact arrangement of the blkindex.dat on disk (locks are acquired per-page).
As there is no consensus even among the same version (unless you can prove the otherwise), this is absolutely a bug, not a rule.
of that bug was also a protocol rule.