if you are wearing Gem leggings (the gravity greaves) and you enter spectator mode as soon as you go through a wall you fall below bedrock at -1 or -2 and you cant get back up without tping out.

(from xJon/Tekkit-2#175, resolved by removing Aqua Acrobatics, reproducible with just ProjectE + Aqua Acrobatics)

Fixed in 1.15.3.