Fall:Out [v0.3.6] \/\/TOP\\\\
Bitcoin v0.3.6 and earlier - OP_VER pushed the version number of the verifier onto the stack. Any use of scripts observing that number would have caused a fork, one per release. As such, every released before v0.3.6 could have triggered a hard fork. However, OP_VER was never actually used.
It wasn't until BIP65 came along and redefined OP_NOP2 as OP_CHECKLOCKTIMEVERIFY which activated on December 8, 2015 and had its first transactions spent using that rule on the same day. I initially thought that this was the point in the blockchain at which pre-v0.3.6 clients would fail to validate new blocks. 041b061a72