The situation in the picture down here does not look pretty and can be avoided. This problem is called pillowing. It is named after the little humps, bumps and holes that occur across the top surface of the 3D-Print. Pillowing can occur when the 3D-Printer is working on the top layers of a 3D-Print. Within a 3D-Printed object, usually a lot of space is empty and is being filled up with infill material. This infill material can be set to none or full depending on how strong and rigid you want your 3D-Print to be. The lower the infill, the larger the gaps are between the support structures inside the 3D-Print. When these gaps become too large, the roof of the 3D-Print will start to collapse and will either cause holes or strange bumps. But do not worry, pillowing is easily preventable.
First and foremost, increasing the amount of top layers can make a big difference when it comes to preventing pillowing. The more top layers you print, the better the top of your 3D-Print will look. Always print at least 5 top layers when printing with 0.2 mm layer heights and make sure your fan is cooling at max speed to cool down the filament before it can sag or even drop into the gaps between the infill supports. Of course decreasing the layer height, means increasing the amount of top layers. The smaller the layer height, to more fragile the layers become. Try to keep the amount of top layers to about 1 mm of total height (with a 0.1 mm height, that would come down to about 9 top layers).
A simple solution to pillowing is to use the cooling fan. Without a cooling fan cooling the filament when its deposited, the filament would stay in its soft state and will sag under its own weight. Therefore it is recommended to cool the filament properly as it leaves the nozzle so that it will solidify as quick as possible to remain rigid and straight. In the picture below, the difference between the use of the cooling fan and without is easily recognized.
Keep in mind that when you set your cooling fan to full speed while printing with a warp sensitive material such as ABS, HIPS or nylon, the chances of warping increases drastically. When printing with warp sensitive materials, make sure to increase the amount of top layers (>10 with 0.2 mm and >15 with 0.1 mm layer height) instead of to increase the cooling fan speed. These kinds of materials cannot handle rapid cooling and therefore require a low to zero cooling fan speed.
The gaps in between the infill support structures are the main cause of pillowing. The top layer filament cannot bridge the two infill support structures and starts to buckle under its own weight. When increasing the infill percentage, the infill support structures increase in density and therefore decrease the gap size. The minimum recommended infill percentage is 12%. Below this percentage, the gaps simply become too large for the filament to properly bridge without sagging/buckling. If you require a light weight product and need to keep the infill percentage low, make sure to use a material that can handle rapid cooling and keep the amount of top layers high.