Having a random flashback of the events that took place while under the effect of some dank marijuana. Usually happens while sober, and the memory suddenly hits you out of the blue, even though you can't remember the rest of the night. Sometimes it's possible to piece together a whole night by putting together everyone's hashbacks.
Example of Hashjacking:
Omg check outthis cool thing or a tweet full of hashtags #NYCMarathon
#PrimarySchoolMemories
#Secondaryschoolmemories
#IveAlwaysWantedTo