Minecraft 1.8, Minecraft 1.8.1-pre3
To reproduce issue you will need:
2 players in game or a command block running on a clock.in spawn chunks in overworld.
In creative mode go to nether with one player, clear out all bedrock at layers 0-5 beneth you so you are just floating above void then run this command
if you use command it still executes correctly
execute @a ~ ~ ~ detect ~ 0 ~ bedrock 0 say hi
What should happen is any person that is standing over this area in the nether that doesn't have bedrock at layer 0 beneath them should never have this command successful run on them.
In the 2 person scenario if person 1 is in nether above where there is no bedrock and person 2 executes the command from the overworld the command runs successful on player 1 even though it should be false.
You can have the command loop on a clock to get the same effect if its put in the spawn chunks if your not lucky enough to have a friend help you test it out.
Suggested ways to fix it.
1. Have execute detect command never work on players in other dimensions
2. Have execute detect commands work correctly across dimensions.
Note: It would be nice to know why this bug happens.