That's what's so scary about it - even someone who is extremely educated, has been a long term user and is very familiar to how they react to different drugs can get caught out. The effects of a combo can also vary from time to time - what hardly affects you one time could be dangerous in another situation due to a whole range of variables. For me it's just not worth the risk - I was caught out myself and I never expected that to happen
Quoting this because it's worth repeating.
Same thing happened to me. I used to use a combo of three things (which I won't mention other than to say they were all powerful downers) on a daily basis. I did that combo for months, sometimes I dosed a little more, sometimes a little less, but I always prepped a dose which I felt I could handle.
One evening I did an average dose of the three after getting home from work (dose was actually smaller than many other doses I'd done dozens of times before). Started feeling that familiar pleasantness, sat down to have dinner and watch TV with my girlfriend and.... next thing I know there are a bunch of people standing over me, calling my name and shining lights in my eyes - calling me back from death.
Why did that happen after dozens, or even hundreds of times with that same combo? No idea. Will never know.
I will say, though, that I felt like an absolute idiot for scaring the shit out of my partner and family - and also a raging hypocrite for being someone who is well-educated and experienced in habitual poly substance use and who has tried to provide this forum with useful safety advice. I would always advise people on here against mixing CNS depressants, yet thought I'd be fine because I had a tolerance and knew what I could handle.
Bottom line: there's simply no fail safe formula for mixing substances - even if you've done it a zillion times before without incident. I wouldn't bother telling people what to do or not do anymore, I'd just remind people that dire consequences can strike at any time with combos like benzos and opiates and that there is no such thing as a 100% safe method of mixing them. No need for fear mongering, but no need to pretend there's a guarantee of safety either. It's a risk each and every time.