I've had it happen. One time I went out to pull some gravs, and my tolerance was REAL low at the time. I took 1 grav and when I packed the second it hit me all at once, I was fucking GONE. So I took 2 more, then I went inside, I either fell asleep for 2 hours, or after 2 hours, or something, but when I woke up I was definitely still high. I was pretty freaked out cause I woke up and still felt it and I was like "whoa, oh yeah I toked awhile ago...but I fell asleep...doesn't that make it go away?!"