If a player passes through a Nether Portal while in sneak mode, that player retains some properties of sneaking after coming out on the other side.

• Cannot open chests, flip levers, etc.
• Visually appears crouched to other players (including nameplate being hidden by opaque blocks)
• Moves at full normal speed, despite appearing crouched
• Falls off of block ledges, despite appearing crouched
• Able to break and place blocks normally

Effect remains until player presses/releases sneak key.

Tested on a Realms Survival world in 1.8.3.