For me, it is always word order. I'm not really a linear thinker, so I have the essence of the sentence if you will already dancing in my head before it spills out on to paper or, in this case, electronic text.
The trouble is the little things get me. Not spelling, but usually word order.
There are also some embarrassing occasions in which I think of two or three sentences at once and type a combination deal down.
No shame in that.