Vocabulary: an event, usually an unplesant one, that might or might not happen

Solution 1:

How about the word 'contingency'. Here's the second of two senses listed on Merriam-Webster's online dictionary:

a.) An event (as an emergency) that may, but is not certain to occur

b.) Something liable to happen as an adjunct to or a result of something else

To be honest, I'm not familiar with sub-sense 'b', and while M-W seems to specify possible (but not certain) emergencies, I've found myself using 'contingency' to refer to minor hiccups and set-backs.

Also, note that 'contingency' also refers to provisions made to address such (possible but not certain) events. Strangely, that sense is not mentioned on M-W, but is listed in the Oxford Dictionary (10th edition; page 307).

Solution 2:

Presentiment An intuitive feeling about the future, esp. one of foreboding And An emotional response that can only be described as precognitive, such as a feeling of dread before an impending disaster.

Words with similar meanings: boding, foreboding, premonition, presage
TFD