i've also experienced some very intense LSD flashbacks on weed, they made me scared and paranoid at first but now i've learnt to not panic (also i'm on valium)...
Yes this has happened to me!
I used to get swirling patterns of alphabet letters all over the place whenever I toked.
It wore off after a year or two.
It was definately to do with a bad acid trip.
This has happen to me before but it happen after I tripped lsd a day ago. I felt unbelieably high and the ground started swirling, turning into fractal patterns, it was awesome I have to say. It brought me back to the tripping headspace for a min