I have never had a bad LSD trip per-se, but if I take two doses in the same week very close to each other I sometimes regret taking the second dose as I get very minimally high and just a headache usually. Being sort of high on LSD due to tolerance sucks, you are just high enough that you cannot think straight but not high enough that you get any significant value out of it. So my worst LSD trip would have to be one of the times I dosed back to back without having enough time in between.