One of the more common ones of late since the US has made it mandatory for all airline crew to attend personal interviews at US Embassies abroad prior to having their class D visa renewed is "failure to obtain travel documentation". The denial rate for crew visas is actually around 10% (meaning that 1 in 10 FAs will have their visa denied at some point - a number that seems high but isn't really when taken in contrast with the 55% worldwide refusal rate for all US NIVs).