If you are using anything other than rosin flux or rosin flux core solder then residual flux can cause a weak short. Non-rosin flux is usually a weak acid flux which is conductive if there is any medium like water or something similar. Rosin based flux does not conduct. The primary advantage of the newer kind of flux is that it is easy to clean. Cleaning rosin flux requires a strong solvent.
Sometimes when you cut the copper you can leave a tiny sliver that will also cause a short.
