A new primer pairs were designed for detection of Vibrio cholerae specific 'ompW' gene using real-time PCR machine ABI Step One. The expected size of the PCR product was 191 bp. Upon PCR amplification there was an initial dip in the derivative reporter reading. Specific peak was seen at around 78 degrees. No primer dimers or non-specific bands were observed after agarose gel electrophoresis of SYBR Green PCR products. What could be the reasons of the initial dip/peaks at the beginning of the melt curve?