Just because you mixed drugs one time and didn't die doesn't mean you are safe; most people who mix drugs do it many, many times before it finally kills them. The safest path is to avoid the dangerous drug combos we listed above and stick to using one drug at a time or safe combos like weed and opiates or weed and alcohol or weed and almost anything.
If you must mix drugs then take extra safety precautions: don't use alone and have a friend with Narcan (naloxone) nearby. Narcan can reverse an overdose even when drug mixing is involved so long as one drug is an opiate. When in doubt, use Narcan!
There is also a huge individual variation in tolerances for different drugs; what might be perfectly safe for you might kill your friend dead.
For more information about drug mixing visit these pages:
Disclaimer: All information on this site is merely intended to save people's lives and should not be taken as a substitute for medical advice.