Have you ever connected an ancestor to grandparent/grandparents but not sure of the parent?

Yes..in fact, one of my main family projects was just this.. the grandfather left a will, 5 sons matched men in 1790 census. To define mine, had to investigate all 5 and the 16 identified grandsons. Was a huge challenge but very worthwhile.

What you can do, assuming you are POSITIVE in your facts... put a ghost son in as child of the grandparent and name him X Jones (x being a temp first name). Then link the child. You can go in later and edit Mr. X when you know who he really is. And, don't forget.. you sometimes solve a puzzle by looking at the neighbors and friends to spot a wife. ESPECIALLY if and when her surname is far less common than the husband.

EDIT

I personally have seen more than one will, in which the person leaves property specifically to a grandchild by name. If the grandchild's surname is that of the person making the will.. the odds are very very good that the grandchild is son of a son. In some cases, the son has pre deceased the father/ grandparent.

You don't list it in your tree.......... you research to find out the line back........... and yes I was informed when I began my research who my great grandfather was ( by my grandmother who was his daughter in law and who knew him) it turned out the man she told me was my great grandfathers uncle ( so the brother of my great great grandmother so she lied and did so to cover things up) so I researched back on the same/correct line for one set of great great etc grandparents but not the other which completely confused me.......... this was before the internet so real research and going to records offices and took me 15 years to sort out via real research......... now and if I knew what you do I would put all the information down but not in a tree and research until I found which one it is.......... so you need to work backwards and find burial, death, marriage, baptisms and births of each 'son' or sibling........ and I say sibling as it may be one of the girls and not the boy like mine was who like my great grandfather used his mothers maiden name not his 'fathers' name........ never assume do the research.............................

All the time. Then I backtrack and filter through the records to see if the person belongs or not.

Sorry, you need to work backwards, one generation at a time.

If you do not know which son is your ancestor, you are merely guessing further back.