Having lived closely with one, I see a lot of what some might call "anger" but is probably more like irritation or annoyance. However there are several types:
1) The exasperated INTJ - happens often when dealing with stupid people that don't understand INTJ's well-arrived at conclusions. Usually involves me getting a shouted one-sided argument in the car on the way home, even though I have no investment in the other side.
2) The you-fucked-up INTJ - happens when I do something REALLY P-like (lose keys, run down the car battery). This will be a simmering anger, which they will deny, but it's pretty obvious that they're upset from the outside because they're acting like assholes.
3) Destructive INTJ - when the INTJ is betrayed or someone really really fucks them over. Watch the fuck out. I find the seriousness of the anger is inversely proportional to the external cues (shouting, etc), so if you see a silent but fuming INTJ you might want to watch out for life and property.
One caveat is that I've long thought that my INTJ is a bit weak in the T, so I don't know if that changes things. Other INTJs may disagree strongly with the above.
Overall, serious anger is very very uncommon. I have made the INTJ angry (1 and 2 at least) and it's honestly not that scary, I know that rationality will prevail and we'll get past it relatively quickly. One thing to note is that apologies don't really work for them - they want you to fix the problem. If you can't, you just have to wait. Fortunately they don't stay angry for very long.