If the user client is well implemented either it or the user notices that an action didn't take effect and tries again, similar to what you would do if a phone call was disconnected unexpectedly or what most people would do if a clicked button didn't have the desired effect, i.e. click it repeatedly.

In many cases it's not a big problem if some traffic is wasted, compared to desperately trying to process exactly all of it in the correct order, which at times might degrade service for every user or bring the system down entirely.