There is a Gemara in Kesuvos (2b–3a) where it says that if someone divorced someone on condition and then didn't fulfill the condition, the woman doesn't become divorced, she becomes un-married retroactively.
There were many discussions how to solve mamzeirim problems. Why not have the man give his wife a divorce al tnai and then not fulfill the condition BeOnes (like tell his wife on 1 Adar that she will be divorced if he doesn't come back by the 1st of Nissan and then be so far out of town then that he won't make it back on time).
From the fact that this idea was never proposed, I suspect that it isn't a valid trick.
Why not?