The mistakes I come across often (particularly earlier into the work) are:

Commas! I used to put commas to connect an independent clause and a dependent clause when none was needed.
  • I still tend to make this mistakes sometimes, when I'm not paying close enough attention.
Maintaining character voice.
  • In this particular story (Emory), it's very difficult to keep his character steady throughout. I'm not sure if this counts as a "convention," but it is something I have to be 100% aware of when writing. His choice of wording and sense of humor, while most like myself, is still something I find laborious to write. I love Emory (and I also love writing about Thayer), but he is an extreme load of work; I'm almost tired when finished writing.
Parallelism
  • Sometimes, in the midst of writing, I sometimes forget to make my sentences parallel. I've grown as a writer, though, and I have come to catch my mistakes as I make them.