I personally am not aware of any Yoshinkan affiliated dojo where there is no possibility of failing a test. I've failed tests myself.
What dojo are these? Do they represent all of aikido, or only a portion?
PS. For me personally the consequences were that I spent quite a bit of time (almost every training session, between 5 and 6 days a week, for about two months) prepariing for the re-test. But hey, that was just me...
My experience is primarily with the two major aikikai-affiliated groups in the US (USAF and ASU). I've seen many dan tests (and a few kyu tests) over the years in both groups and have never, to the best of my knowledge, seen anyone fail a test. I'm told that decades ago failure was much more common in both groups; in some cases even everyone testing would be failed. I guess they finally realized that failing people was bad for business, so instead they started making the time in grade requirements longer (especially in the case of the USAF, which on paper requires nearly four times as many practice days as the aikikai hombu does for shodan) and then just passing everyone who tests.
I've also been in dojos where a significant amount of time from every class was spent helping one or two people prepare for an upcoming test that they were sure to pass anyway. This preparation would usually begin several months ahead of the test, possibly even six months or more in the case of dan tests.