Where dose these unknown colonies come from? - (Jul/15/2008 )

now I am cloning a 10kb plasmid
insert is 2.9kb
vectror is 7kb
both of them show sharp bands in agarose gel after restriction enzyme digestion and colunm purification.
however, after ligation...the LB plates grow so many colonies,
but none of them is positive, even not vector self-ligaiton.
(the plasmid from these weired colonies is about 2.5kb).

I've checked my competent cell, my LB plates, my electro-cuvette...no comtamintion!!
I don't know where these colonies come from, so I even don't know how to avoid them.

dose anyone have experiences of this strange problem?
it could be great to give me some adivces to get rid of these...

Are you selecting transformed colonies in any way? Say Ampicillin, Kanamycin or some other strategy?