Reduced intensity of the fluorescence signal in the amplification curve was observed when using a WHO recommended real time RT-PCR for influenza virus detection. A single mutation, G189T, in the conserved region of influenza virus matrix gene was detected by Sanger sequencing. The mutation is located in the probe binding region, hence we speculated it could be the reason for the atypical shape of amplification curve. The mutation was first noted in Slovenia in 2011 and 2013 for seasonal influenza A virus types A(H1N1)pdm09 and A(H3N2), respectively. In the following years, 2014 and 2015, the majority of influenza A(H3N2) viruses alone carried the mutation. The amplification of matrix gene for these influenza A(H3N2) viruses continuously resulted in the atypically shaped amplification curves. The performance of the particular assay was critically affected; therefore, the assay was no longer usable as diagnostic tool for influenza virus detection. Mutations in the conserved region of influenza virus genome are more common than expected and this would need to be considered when targeting matrix gene.