There are a few reports of LSD overdose. The primary one that comes to mind involved a mix up between crystal LSD and cocaine. The user sniffed over a hundred milligrams of pure LSD crystal, bled out their skin and died. However deaths from LSD are freak accidents. In the normal course of events, you will not overdose on LSD. It would cost thousands of dollars for the average person to obtain enough LSD to have a shot at killing themselves. If you look at Erowid you will see the estimates for LD50 are somewhere around 15mg I believe (it has been a while since I looked). This is considered a conservative estimate though, and there are many reports of people taking more than this and living. If you eat a sheet or less you can be pretty confident that you are not going to die, and if you can count the number of 100ug hits you have taken on your fingers and toes, you are not going to die. I personally would not feel safe taking a thumbprint though. I have heard stories of people taking 25+ mg of LSD crystal and living, but I think they are indeed risking potential death at such high doses. On the other hand, the average dose is about one tenth of a single milligram, so the people taking 25 mg doses are taking 250 times as much as most people do.