If you're focusing on fat loss, not much can be more discouraging than seeing a spike on the scale. While I don't like to focus only on the scale for many reasons when it comes to progress, certainly I can understand and relate to this occurrence.
One question I get asked often is "am I gaining muscle versus losing bodyfat?" I do believe that this can be a more long term answer, but in a short term environment, this isn't likely the case. Anyone who has been working to put on muscle (esp. naturally) can vouch that this process takes time.
However, there is a closely related, but different scenario that can create a sudden spike in the scale. If someone hasn't trained for awhile, perhaps changed their training focus or maybe had their intake dropped back (specifically carbohydrates), there could be a depleted effect on muscle mass. This is due to glycogen (stored carbs in the muscle) being low.
When the muscles start to be used again and/or carbs are reintroduced, these stores will get replenished. When glycogen is stored, fluid is stored within the muscle cell as well. So even though there might not be a big spike in muscle growth per say, there could be a sudden spike in muscle volume, which in turn, can spike the scale. This is also why bodybuilders and other athletes will 'carb up' prior to a competition.
Again, there are tons of reasons why the scale can mislead, but if the scenario above describes you or someone you know, rest assured, this can be a good thing.