Some years ago a couple of ISTJs caused a realization in me that basically all of their joke remarks are based on truth. It's just that whilst putting out their joke remarks, they are also being completely clueless that they are actually sharing their deeper thoughts; the things they wouldn't otherwise say. They don't realize that joking about it reveals their inner thoughts. I'm sorry I don't have any examples right now... I used to have... what a worthless post, huh...
Yea, the first time along I do a double take and instantly analyze it,, as in, where did that come from? The second or third time, I start thinking of what it is that I am projecting to this particular type/person/generation. The third or fourth, I come to your conclusion about them revealing their inner thoughts but am not too sure that they are that clueless...
ISTJs are also good at killing good mood and inspiration with few uncarefully selected words. Consider the following cheesy dialog as an example:
Person: "Look, I bought a new sofa

"
ISTJ: "The fabric is torn under there."
Person: "Yeah... it was cheaper. Well, look at these new curtains."
ISTJ: "They don't match."
Person: "A-ha..... I have a new phone

"
ISTJ: "Hmm... that model doesn't even have a camera."
Person: "Right. Whatevvvv!!!

"