Influenza viruses are a continuous public health burden and pandemic threat. This is in part due to their rapid rate of evolution, caused by errors introduced by the viral polymerase during replication. Insertion and deletions errors in particular lead to major changes in viral proteins. The influenza genome presents insertion/deletion hotspots and here I will focus on a human influenza hotspot allowing the virus to escape immune recognition. I will use a novel and sensitive approach to study the mechanisms underlying this phenomenon. A better understanding of how these errors occur might allow us to anticipate them in the future.