We've had such a case a short time ago (months?) in the news. Does anyone remember what that was about or even has a link?
I was considered for (but ultimately did not qualify IIRC) a trial of a new Hodgkins treatment. If I had qualified and opted in all that would have meant is basically they toss a coin, heads I get the new treatment, tails I get the existing "Gold standard".
As well as not using a placebo because it's a death sentence, they also can't truly blind the trial because the new regime is different drugs, with different side effects and a different calendar. The staff must know which they are giving you and even if they didn't just tell you it's not hard to look up online what the Gold Standard is and tell if you aren't getting it.
So all they have left it's a two arm trial in which they've tried to match the arms up, and then compare the results.
In my case the eventual trial results show the new regime was slightly worse, not better as hoped. Slightly more (statistically insignificant but not good news) deaths and worse side effects. Unfortunate, but there was only one way to find out.
It actually can happen quite frequently. Trial protocols can pre-specify early stopping criteria: if we see drug A is better than drug B by a factor of X at time T, then we can declare superiority with given statistical confidence.