People often try too hard to be grammatically correct and end up making a hash of it. The common example of this is him and me in the objective case which is hypercorrected to him and I. We have a faint memory of being rapped over the knuckles for saying me when we should have said I and end up nervously putting it everywhere.
Recently I heard a radio presenter, afflicted by such grave doubts, hesitate over the past tense of leapfrog. He ended up with the rather bizarre form leaptfrog as in They leaptfrog over the competition.
It seems that the notion of leapfrogging comes from a game that children have played since the 1500s. Originally it was a boy’s game since girls in dresses would have found it a little difficult. One child was the obstacle, usually crouched on the ground. The leaper placed his or her hands on the shoulders of the child/obstacle and jumped over, spreading the legs and feet on each side of the obstacle in the manner of a frog. This is the important bit. It is not so much the leap itself as the spread feet that makes it froglike.
The first citation is Shakespeare in 1600 and is a reference to the game. The first figurative use is 1704.
So now we get to the past tense. The most sensible thing to do with compounds is not to reanalyse them and pull them apart but to treat them as a whole word and do the various forms accordingly. So leapfrog should have leapfrogging and leapfrogged as its present participle and past tense. I suppose that when the first element of the compound is a verb it has a much greater power to attract attention which is why there is a reasonable amount of support for leaptfrogged, often spelled leapt frogged. Not that I think that improves it. But leaptfrog is just not going to pass muster.