Regular blocks can not be placed manually at Y=255. You can however, place any blocks that need to be attached to a block, such as buttons, sugarcane, or torches. The setblock command also works, as do buckets. You can also push blocks to Y=255 with a piston.
MC-69878
Solid blocks can not be placed at Y=255
Resolved
Duplicate
3
KingSupernova
(Unassigned)
Unconfirmed
Minecraft 1.8-pre3, Minecraft 1.8, Minecraft 1.8.1, Minecraft 1.8.2-pre1
Retrieved
The most minor glitch ever, but I noticed this and hope it gets fixed.