1. Use unsuitable tools
Trying to screw up / unscrew a screw with unsuitable or ruined dimensions keys does nothing but make the operation more complicated. The risk is to strip the screw in question and then make the extraction impossible. For each element there is the right tool, use it!
2. Wash the bike with the high pressure cleaner
High pressure water shortens the life of the bike components. It can get to wash away the protective grease inside the key points of the frame leaving the metal in contact with other metal. The high pressure cleaner is to be avoided, it is best to use bucket and sponge or low pressure water can.
3. Use too much strength
Modern carbon bicycles are very delicate so it is good to use a torque wrench to fit individual components. Tighten too much on carbon can create damage or scratches that in the long run will cause the piece to break.
4. Assemble without grease
Between two pieces of metal must always interpose a lubricant. Caution: Using unsuitable greases and lubricants (such as those for car) does not help. It is best to buy bicycle-specific products that are water-resistant and naturally active at room temperature.
5. Put too much lubricant on the chain
The oil-impregnated chain is a perfect receptacle for dust and dirt that, in the long run, wears the meshes. Never exaggerate.
6. Mechanical Ignorance
Before get your hands on the bicycle make sure you have a clear idea of the steps to do.
7. Install the pedals on the contrary
The pedals of the bike (right and left) have different threads and so they tighten differently. Right in clockwise direction, left counterclockwise. With a wrong assembly the pedals will be off their axis making the bicycle unstable.