At least since snapshot 13w39b, weighted pressure plates activate when a player steps on them. I have not tested any previous versions, and as far as I am aware, this is not intentional (weighted pressure plates are only supposed to activate with items ontop of them).
MC-34829
Weight pressure plates activate with player on them
Resolved
Duplicate
1
Sen
(Unassigned)
Unconfirmed
plates, pressure, weighted
Minecraft 13w39b, Minecraft 13w41a
Retrieved
Just found this bug. Along with MC-29778, this makes weighted pressure plates practically not useable 😞