Incorrect placement of adverbs in a sentence.  The pronunciation of "ed" at the end of a word. t also depends on the level of the student.

Romelle, I agree with you, the verbs make and do are very difficult for me.

From what I noticed 

Make/Do – In Spanish, the verb hacer means both “to make” and “to do.” This leads to sentences like “I make homework” instead of I do homework.

A mistake I see repeated time and time again is people writing "is" instead of "it's". 

In writing I'd have to say that by pure volume, it's punctuation. Spanish tends to run on with lots of commas "," and lots of dots... and it's cool start a sentence with and or but. English is very different in terms of phrasing, cadence and punctuation so often while correcting Spanish speakers writing, I have to correct the punctuation multiple times in each sentence and figure out how to make it flow properly in English. It's often quite a challenge for me. Even advanced students still seem to have some problems with this aspect of switching from Spanish to English thinking and writing styles. 

The good thing is that it has taught me how better to write in a Spanish style! I just do what I correct in the English essays in my Spanish essays and they are better. :D