I know I've brought this up before, and didn't receive much (any) constructive input to the topic. So, let's try again (this time with concrete RL examples)!
Psychological disorders must confound the typing process, making it difficult/impossible/reducing overall certainty of type. Case in point, I used to be 90-95% sure my girlfriend was ISFj. She has rapid cycling bipolar II, we were in a long-distance relationship for 6 months of our 9 month (ongoing) relationship, and we have been living together for about a month. During the last few months, I've become less certain of her type. It's supremely difficult to differentiate her "normal" state from her hypomanic states, and I'm not sure I've witnessed a "normal" state in her; she was in a depressive episode for the first 5-6 months of our relationship, and has been more or less hypomanic since. Because we started dating during her last depressive episode, I had only that state to utilize for typing purposes. Now that she's been hypomanic, our interactions are not that of activity, but of some unknown interaction (and I'm not willing to attribute that change to the changes in our relationship, i.e. being POSSLQ and no longer in a LDR, because I noticed the change while we were still in a LDR). She's classically hypomanic, she doesn't sleep much, she's very irritable, she makes extravagant/impulsive plans, and seems always on the verge of making a major and unnecessary purchase. She seems very much unlike an ISFj, although I cannot at this time suggest an alternative typing.