From Glitch City Laboratories
The Focus Energy Glitch is a glitch in Pokémon Red and Blue Versions involving the effect of the move Focus Energy.
Focus Energy is a move that, ever since Generation II, increased the user's critical hit rate. This glitch, causing it to sometimes do the opposite, was fixed in Pokémon Stadium.
Focus Energy's original effect
After Focus Energy is used, any move that does not have an increased critical hit rate (Razor Leaf, Slash, Crabhammer, Karate Chop) will have one quarter of the normal chance to land a critical hit.
As for moves with increased critical hit rates, the reduction is dependent on the user's speed. If the user is faster than the opponent, no reduction occurs. If the user is somewhat slower than the opponent (exact numbers still need to be found,) then its critical hit rate will lower by 50%. If the user is significantly slower than the opponent, then it will become impossible for the user to land a critical hit.