If you have a strenght-potion with lvl 149 it affects like a weakness-potion lvl 149. Also when you use a speed-potion lvl 149 your window will turn around.
When you hit someone with a strenght-potion lvl 49, you see, that the other one is "dead", but he can still move around but the dead-message comes instantly and for example the scoreboard shows one more dead.
Duplicate of MC-10755 and MC-15145.