floppy tail syndrome (or fts) is when the vertebrae at the base of the spine that supports the tail become weak causing the tail to "flop" either to the side or toward the gecko's spine when it hangs upside down. this can happen because the gecko isn't getting enough calcium in its diet or because the gecko hangs upside down or face down a lot and the weight of its tail weakens the vertebrae.

it is actually quite common among crested geckos and although it doesn't really affect their health in any way it looks quite odd to see their tail hang that way when they are in a face downward position. the tail makes up 10% of the weight of a crested, so when you think about it, that's a lot of weight for that one vertebrae to support if the gecko hangs upside down a lot of the time. because in the wild cresteds always drop their tails it is thought that this vertebrae may have never strengthened or evolved over time to be able to support a tail.