When it comes to using scale factors, I've noticed some common mistakes that can confuse students. Here are a few:

1. Mixing Up the Ratio: Sometimes, students think the scale factor is just a plain number. But it’s actually a ratio! If you have a scale factor of $k$, think of it as $k:1$.

2. Different Units: Not using the same units can cause problems. Make sure you always check that the units match while you work.

3. Forgetting About Area and Volume Changes: Students often forget that area and volume change in special ways. If $k$ is the scale factor, then the area changes by $k^2$ and the volume changes by $k^3$.

These small mistakes can make things much more complicated!