I have worked with two primers: LCO1490/HCO2198 and dgLCO1490/dgHCO2198 for the COI gene in the same specie. But I get different information using both.
Different pairs of primers can amplify a different sized product from the same template DNA.
I can also tell you that COI and other "universal" or "barcoding" primers will often amplify DNA from an unintended source (e.g. primers for 16S designed for fish will easily amplify human DNA).