Oh man, I"ve had this happen too. Multiple times!!!
Once in my 10 gallon with store-bought top fin dividers, another in another 10 gallon with DIY dividers, and the most recent time in my 2.5 minibow that I had divided with the divider it came with (as QT not permanently)
Fish can wriggle through the smallest spaces, kind of like mice do.
My suggestion:: Grab the sides of your dividers that are against the tank walls, and pull them from the mesh as hard against the tank wall as you can, in all areas. I have started to do this in all my tanks after water changes and haven't had another slip through since then. -- I know it sounds really obvious but some people dont think about it.