The biggest dose of LSD ever medically documented as taken by human beings occurred on the 29th of July, 1972 in San Francisco. It is documented here on The Lycaeum.

In this fascinating misadventure, eight people, four women and four men, ages 19 to 39 (to me these numbers just shout "sleazy old guys and young girls"), came across a white powder which they assumed to be cocaine, and snorted two lines each. It was in fact LSD, 80%-90% pure. The amount ingested is unknown, but was estimated to be several milligrams, each milligram being 10 times the usual dose of 100 micrograms. Within 5 minutes they realised their mistake.

When they arrived at San Francisco general hospital around 15 minutes after snorting the LSD, five of them were comatose. The medical problems that they experienced are various and sound dangerous, including "respiratory arrest" for three of them and "aspiration" for the two others. It's a fair guess that these people would have have died without medical help. However "Recovery was rapid and complete by 12 hours, but included a stage of hyperactivity and severe visual and auditory hallucinations." No kidding. All left the hospital within 48 hours.

The abstract of the original doctors' report can be found by searching PubMed (or just Google) for "Klock JC, Boerner U, Becker CE: Coma, hyperthermia, and bleeding associated with massive LSD overdose: A report of eight cases. Clin Toxicol 8:191, 1975".
Erowid has the full text of this report here.

Though the LD50 of LSD in humans is not exactly known, it is estimated to be in the range 0.2 mg/kg to over 1 mg/kg, i.e. for an 80 kilogram person, between 16mg and 80mg.

LSD has however been implicated in suicide, homicide, accidental death and other dangerous and odd behaviour at usual doses, not to mention trouble with the law. Know your drug, know your limits and know yourself.