The 1967 off-metals are probably the mint workers having a bit of fun as this coincided with the closure of the mint at Tower Hill and its relocation to Llantrisant. There are quite a few of them about.
The 1868 cupro-nickel proofs were intentional as all denominations otherwise struck in bronze are known and it coincided with the introduction of cupro-nickel coins for Jamaica. Copper and bronze don't perform well in humid tropical conditions, so they switched to cupro-nickel for this reason. The same applied to a number of British possessions with similar climates.
For example, here is a third farthing struck in cupro-nickel from the same period.