I will give you a very good example:~
I am not implying in any way that the catastrophe was known in advance of the event.
If you look at a daily chart going back to Sept 11th and previous. You will see that the market was already falling owing to weakness in the backround that had not been resolved. Sept 11th was so to speak, if you will pardon the pun, the last nail in the coffin. As a result of events like these having world wide implications, the formation (not a pattern) was replicated right across the world in nearly every instrument.
Occasionally markets are caught off the hop. When Nick Leeson was buying the Nikkie, the market was weak, unbeknown to him for similar reasons as above. Suddenly, in Japan, there occured an earthquaque.
The fall paused, and rose a little, for 3 days and then collapsed again.
In 1998, the US markets were in an intermittent ambivalent mood. One evening, prices were falling owing to lack of demand. Suddenly they rocketed skywards without warning as a result of an unexpected cut in US rates, announced late by the Fed.
There are situations in which the price can be discounted in expectation of future events, but it is not a hard and fast rule, because some events cannot be anticipated and therefore responses have to be immediate, whereas events (whose outcomes) can be anticipated lend themselves to prices being adjusted in advance.
The problem is that this discounting is masked frequently by supply demand schedules that run parallel to main events. This is why sometimes it may appear that the event was known or even planned, but it is not the case.