“I did not see the real significance of that wreck at once… Certainly the affair was too stupid… to be altogether natural…” (pg 20)
An example of foreshadowing, it is easy to find several examples of it throughout the novella since Marlow is telling his story from a looking-back standpoint. The fact that he mentions how unnatural the wrecking of the steamer lets the reader know that there is no way that it was unintentional.
EDIT: It is important that Conrad made it apparent that the wreck was intentional because it alludes to the fact Marlow's presence in the Congo is not exactly welcome or kosher. It foreshadows the future troubles Marlow will have with the Company members.