In the Star Trek: Voyager episode, "Threshold", they discover a "new kind of dilithium" which allows them travel at infinite speed (Warp 10). There are hopes that this would allow them to return home in an instant.
But then for some reason it turns you into a lizard.
But then, the Doctor comes up with a way of turning you back into a human - and Paris and Janeway (the two who underwent these transformations), in the subsequent years in the Delta Quadrant suffer no ill effects of having been a lizard (at least none worth mentioning in any episode), not counting a few abandoned baby lizards - which would seem to be an avoidable consequence.
So why didn't they just use the technology to instantly transwarp home? Then have the doctor turn them all back into their correct races?
I know there's kind of informal talk about "decanonising" this episode and regret by the writers about it (Is the Voyager episode "Threshold" considered canon?), but this doesn't seem to be official and it's hard to believe that this was the main reason why it was never used, because in the episodes soon following (e.g. in the same season) I don't think they would have held such a low opinion of it. Is this the only reason?