I read this short fiction (short story or novelette) at least 20 years ago.
As in one of my other questions there is in fact no actual element of Fantasy, but it was in a F&SF collection (which might have been much older).
It takes place in Prague. I just went through a time-line of Prague and "Battle of White Mountain" stroke a chord. So it was after the troops of the Holy Roman Empire retook Prague in 1620 after defeating one more revolt.
Two men are sentenced to death. But the executions did not work out. Not just once but several times. The weather is unseasonably cold, even for winter in Prague. Once they should be hanged, and the ropes break before the necks of the men. Because they were frozen and lacked elasticity ? Once they should be drowned, and the Vlatlava has frozen solid. At least one, maybe two more executions fail, I forgot the details. But the story end with one last double execution that finally works and the death of the two men.
Now the most important part of the story is the discussions that the two men have in their prison between the many execution attempts. All the failures have a rational, natural explanation. But unlikely to happen, even once. So several times in a row ? Is it pure luck? Divine intervention? Witchcraft?
It is clear that none of these men has made a pact with the Devil, but maybe someone did it for them ? Or the Devil, even without being summoned, just for making trouble ?
This succession of potentially natural, but very unlikely, events is the only part of "Fantasy" in the story.
They consider lots of hypotheses, but at the end they are executed. I do remember that I did not see the point of the story.
So of course I will "accept" an answer with just the correct title, but if anyone can try and explain to me the point of the story, I believe the rules are that I can give a bonus (fifty points only !) even if the correct answer is already given, for a satisfactory (to me !) explanation.
John Rennie has found the answer : Experimentum Crucis by Rivka Jacobs.
He proposed an explanation for the point of the very end, the choice of an execution method that did not fit the list mentioned in the letter sent by a Jesuit
"Utere jure tuo, Caesar, servosque Lutheri. Ense, rota, ponto, funibus, igne neca.”
"Use your right, Caesar, and the servants of Luther. Kill with sword, wheel, bridge, ropes, fire."
"bridge" translated by "water" in the story, understood as throwing the into the river from a bridge
Execution by squad was not mentioned, so the point is, if these methods did not work, better kill them by another one than give up.
But this does not address the discussions the two men had in their prison, which constitute a sizable fraction of the story.
Can anyone offer a better explanation ?
I thought I could offer a bonus even after the questio nwas answered, but I cannot find how. Was I mistaken ?