How often do you smoke? Occasionaly, or are you a regular?
If last weekend was a one off, it should only last 43-72 hours in your urine.
If you smoke often, pardon my french, but you're probably fucked (proper fucked) as it's detectable for up to 3 months (obviously depending on how often you smoke, how much you smoke, etc).
If they are running a hair test... It's a good kind of indication of what type of employer they are, and I'd probably back out now. Err, if it actually is a hair test, let's just say there is 60 days of cannabis history imprinted in each inch of your hair... So that'll be the end of ya.
The following substances and conditions have been known to produce false positive results for THC tests...
Ibuprofen; (Advil, Nuprin, Motrin, Excedrin IB etc)
Naproxen (Aleve)
Ketoprofen (Orudis KT)
Promethazine (Phenergan, Promethegan)
Riboflavin (B2, Hempseed Oil)
Kidney infection (Kidney disease, diabetes)
Liver Disease
Dronabinol (Marinol)
Regarding LSD... Detecting LSD in urine is difficult and expensive. It's not usually tested for unless the subject is suspected of using it, and it's only detectable for 24 - 72 hours. There aren't any substances which can give false positives unfortunately... Anyway, I don't think you have to worry about that one.
Best of luck