I couldn't agree more with Xorkoth. Difficult experiences are very different from a so called bad trip. These difficult experiences could definately lead to a bad trip, but typically if you ride out the waves and can get yourself back on riding positive waves and recenter yourself, you'll be good.
I've done LSD numerous times, about 15...I've had many varied experiences depending on the strength, type of lsd and of course set and setting play a huge role. Most of my experiences have been positive, fun, enlightening trips, smoking pot in various amounts at various times during an lsd trip. I do have to say it can go either way. The trips where I smoked constantly, there would be brief moments of difficult experiences/ troubling thoughts/anxiety but I could almost always ride out the waves and recenter back on the positive train very quickly without getting too deep into negative thinking. I don't recall having any difficult experiences when I didn't smoke, very brief if any.
My last trip a month or so ago, I only smoked once. I hadent been smoking as much as I normally do either. This trip was by far one of the most intense lsd experiences I've ever had. I had an amazing time and got a lot deeper than I ever have previously. Many amazing, intricate thoughts about life and how spectacular this universe we live in is. Many depths to this trip. I usually trip with my brother and his gf. This was a home trip which we've done together before. Typically we stay together the whole time doing various thing, movies/shows/walks/talk loops/many various thing. However, this trip I drifted off into my room for what seemed lk many, many hours. I have a very mellow room with comfortable lighting, candle going, led burger light sign. My room just seemed very perfect for this trip at the time, where my brothers was a bit overstimulating with his action figures and various "things" he has, which mine is very simple.
Anyways I digress...so I was listening to music on these colored, water dancing speakers that move to the music and make awesome colors on the ceiling/around the room. I layed on my floor and was chain smoking cigarettes and staring at the colors on the ceiling, going with the flow of the music and getting lost in my thoughts. It was AMAZING and very inspiring. I then decided to roll a joint, after realizing I hadn't smoked any pot yet. Took me a little but I got it done. I took two small hits and could feel it increase the effects of the lsd. At this point I was probably about 5/ 5 and a half, maybe 6 hours into the trip. I was still having an amazing time after smoking and still feelin good vibes. I listened to a few more songs and a few more cigarettes before I decided to go back and hang out with my brother and his gf (they're also trippin). They were watching off the air and were going to be putting on the eric Andre show which we've watched many episodes of eric tripping before and I love it. At this point, even though the couple hits intensified the lsd a little, the peak seemed over and the comedown was starting, although the comedown comes in waves, so it feels lk it one min and then is intense the next, so it's hard to tell. As my brothers putting on eric Andre, I lit up the rest of the joint which was fat and most of it left. We were passing it around, they took a few hits but then were good cuz my bro wanted to light up some gravity bong hits for himself (grav bong hits are a usual thing in my house) and I just kept smoking the rest of the joint. I ended up smoking the rest of it, over half to myself. He lit up a grav for himself, then his gf asked for one and he lit one for me too even though I didn't really want one, he thought I did so I just smoked it, not really thinking much of it (didn't want to waste it lol)...Then we were all just chilling watching eric Andre and maybe a few mins later (not sure how long, times hard to tell on lsd) I starting riding down a bad thought wave. It started with something on eric Andre, it was an episode I've seen before but I forget exactly what started this thinking but I felt lk I was tripping so hard and I was just getting freaked out. I tried to talk myself out of it but this was the first time I wasn't able to. I had to get up and leave and go in my room to breathe...I started getting really panicky. I tried smoking a butt and calming myself down but couldn't get off this thought loop. Then some stupid Spotify commercial came on my speakers with a crowd noise came on and it just didn't sit well with me. I stared at myself in the mirror trying to reassure myself of this extreme feeling I was experiencing. I decided I had to take a Xanax cuz I couldn't stop it. Right after taking it, I sat on my couch and lit up a cig. A very mellow song came on. I still had this feeling which I can't explain in words but it was a very difficult experience/feeling. The song and the feeling I was getting felt like the universe was speaking to me and giving me signs about my life. It was the craziest thing. I decided to change into comfy clothes and as I was changing, everything just seemed to line up, the feeling, the music, the universe and the way I was moving. The only way I can describe it is that It was speaking to me. Idk what "it" was but it was something.
After I changed I sat back on my couch, lit up another smoke and my thought instantly latched on to the feeling I had earlier in the night lying on my floor. I then felt and extreme feeling of relief and amazement of what I just experienced. My thought train went back on a positive wave and I was enlightened by the whole experience. Then about 10 mins later I felt the xanax starting to work and was pissed I took it because I had pulled myself out of that difficult experience before the Xanax even took effect, so all I did was dull my trip early ( the headspace not visuals) when I didn't need to if I had just waited out the experience. I jumped the gun.
Overall, however difficult this experience may have been and definately could have led to a bad trip. It turned into being one of the most profound experiences I've ever had and I wouldn't take back that difficult time or feeling for anything. It was hard to go through but it's helped me in the long run. My anxiety has been practically non existent and my life struggles just seem completely small and very easily managed since this experience. Whenever Im struggling, I bring myself to how I felt and experienced the two extreme ends of the spectrum that night and tap into how I brought myself out of it and it comforts me. I wouldn't take back that extremely difficult experience for anything. It has been life changing and something I will never forget for the rest of my life.
To answer your question, marijuana can go either way with LSD. Weed tends to typically make me on the more negative side as well. I can get panicky/anxious with certain strains, but I don't mind that. I find that difficult experiences can be effective at helping and improving your life in the long run. How you view experiences and what you take from them is all in your perspective.
Hope this helps, sorry for it being extremely long, couldn't stop typing once I started going over this experience in my head. Be safe and Enjoy your trip!!
-E
I advise you not to smoke weed, or if you do make it 15% of what you would normally smoke, LSD really synergises and accentuates a lot of weeds effect and vice-fucking-versa. 
