Band appear and disapper in the negative control ?? - (May/19/2010 )

I did PCR on genomic DNA with primers and reaction conditions used succesfully before. I loaded 10ul of a (50ul) PCR reaction to a 2% agarose gel in 1X TAE with EtBr. Ran gel at 100V.

I checked my gel to see if PCR was succesful about 10 minutes after loading. Bands were visible on both samples and negative control . It had not run long enought to be able to read the ladder. Placed the gel back in the box and ran an additional 25 minutes. Checked my PCR on the UV box again and my PCR bands and the ladder still present, but the band i saw on the negative control has disappeared (which is somthing good for me). Can anyone help me identify what could have happened? Thank you in advance.

it happd to me several times but i went forward and did the cloning came out fine in sequencing

primer dimer maybe. What size is your amplicon?