On the first retreat, yes, because I wasn't prepared for such a radical transformation. Some of it was pleasant, and some of it was very unpleasant. Yes, you can get bad trips.
On the second retreat, no.
You could purposely slightly change the instructions to take detours into all kinds of things, and it seems that most people do, but it probably isn't safe and it isn't the point at all. The faster you can get over the "wow, I could do that without any substances?!" stage and focus on actually watching the layers of your psyche unfold as your mind calms, the better off you'll be.
Nothing really bad, just unpleasant states of mind. Brief lapses of panic, paranoya, etc. This is why meditation isn't recommended for people with history of mental illnesses.
On the second retreat, no.
You could purposely slightly change the instructions to take detours into all kinds of things, and it seems that most people do, but it probably isn't safe and it isn't the point at all. The faster you can get over the "wow, I could do that without any substances?!" stage and focus on actually watching the layers of your psyche unfold as your mind calms, the better off you'll be.