Bad Spanish corrections

Sometimes, I see very bad corrections in Spanish publications. As Spanish speaker, I feel sorrow about that people who correct without knowledge of the languaje. I think that all the Spanish natives can write, but not all have the knowledge to correct.
For example, there are very well entrances in Spanish of people who is learning my mother tongue, but underneath of that texts we can see corrections that change the sense of the sentence of the idea (mostly in the cases of prepositions).
If I were Spanish begginer, that bad corrections could give me bad learning.
So, Spanish students, be careful with the corrections that have your entrances.