Interference is a phenomenon commonly seen in those whose memories overlap or attempt to learn similar subjects, such as romance languages. It can also be as common as recalling the wrong concepts than the one you intended to. Interference has many forms, but is mainly seen in proactive and retroactive interferences. This is where information learned earlier interferes with information learned later, and information that was learned later on interferes with information learned earlier, respectively. This is a testament to how the brain making memories is a constructive process rather than a one-time recording. Memories can be changed and emphasized over time much like a book being written. While remarkable this constructive has pitfalls in that we tend to filter or fill in the gaps of our memories with little to no regard for accuracy. This is the origin of effects like interference, retrieval failure and the misinformation effect.
I have personally noted the effects of interference when attempting to learn multiple romance languages. As a junior in high school I had studied French for about seven years, making recalling it almost as automatic as English. However, I ruined this by attempting to learn Italian as well. On a major French exam, I remember writing what I thought was a cohesive, and well written essay, only to find out later that I had written the entire paper in (very poor) Italian. This is a prime example of retroactive interference, where information learned later overlaps and interferes with information learned earlier. This effect was probably amplified by the fact that I studied both languages often, and back to back during my day causing my brain to mix the two together. Interference is an interest subset of retrieval failure in that the brain mixes up what concept it is supposed to recall. I was unable to recall french because my retrieval systems “thought” that the italian encoded in my memory was french.