Writing No-Nos...

• In middle school, I was taught that when using ellipses, only use three!  In fact, overusing any kind of punctuation is a terribly wrong thing to do when writing.  No one wants to read through a dozen things in parenthesis, a sentence with a million question marks and exclamation points or a sentence with colons and semi colons out the wazoo.  
• When asking "where are you?" or "what time is it?", there is no use for the word "at" added at the end. Ex. "Where are you at?"... my mother used to answer me with "behind the A-T."
• I was taught in high school to avoid sentences that begin with "the" as well as "but."  I frequently replaced "but" with "however" though.
• Generally, I am a very wordy person when I speak as well as when I write.  I was corrected by my AP language teacher very early on in high school.  She told me to say what I needed to say in the least amount of words possible.  No one wants you to get to the point in a roundabout way. This awful habit includes my tendency to be redundant.  Repeating things is often unnecessary, even if I think I haven't explained it well the first time.  I just need to fix the first go around instead of adding a second.
• When I was very young, I was taught that all ending punctuations go INSIDE the parenthesis if there are any.  I had a lot of trouble with that one when I was young.
• Subject and verb agreement! This needs no explanation.  But I unfortunately did not learn this until high school. I had poor middle school grammar education.
• The above bullet point is an example of a bad habit I still run into- thinking fragments can be sentences.  They cannot be sentences, ever.  
• Spelling hasn't really ever been an issue for me, but I have run into times when I thought I knew what a word meant and was corrected because I was entirely wrong and it made my sentence and idea incorrect, as well as confusing. 

I'm sure there are many more rules that I have been taught over the years, but I suppose they are so far burnt into my brain that they are second nature and therefore not at the surface of my mind.  Nevertheless, these are plenty to go off of...