When a man is dying, they ask him in a roundabout way whether his wife will be subject to yibum. When he says she could marry a kohen gadol means she doesn't have any need for yibum. But what about when a man comes to divorce his wife, he is considered reliable with regard to his wife's status - sometimes! Sometimes, he is explicitly "not believed" with regard to his wife's status. Which leads to a serious discussion about protecting the women, and also held in a respectful manner that allowed for different opinions. Also, if a will is written on one's deathbed and not yet given to anyone, and then the person dies -- that will is not considered valid. Likewise, a deed of transfer that is to be written on one's deathbed is not valid, because that kind of contract is only effective if the original owner is still alive.