The problem: A lot of relatively easy tasks are often do not bring the results in the expected or desired quality.
It's the "human factor" - one of the reasons (apart from the costs) that drives companies to automate processes. The robot doesn't get distracted and works all day long with the same accuracy.
The reason: Attention and accuracy of people varies because of very dynamic event driven brain activity.
People get distracted by external and internal events that I like to compare with interrupts on a computer. But the difference is that a computer switches attention completely and does completely one thing or completely another thing. Humans can do one thing but pay (more) attention to something else. A further effect is that we often and easily forget things.
The solution: Get aware of changes in your attention, control your event filter, change your environment and support yourself with notes and checklists.
It is possible to get some control over the distractions. First step is to be attentive on what decreases your productivity most. Then you can change the environment (get to a less noisy place for example) but you can also adjust your sensitivity how you get aware of interrupting events (sometimes this is happening automatically - I for example do not hear the computer noise any more if I do not explicitely pay attention to it). Helpful for adjusting your event filter is for example to get conscious about the current priorities and make a clear decision about what you definitely want to do now and what is definitely not important. As our brain does not work perfect a big help is taking notes, creating checklists for common tasks, best practice documentation and so on.