Whether you're an English speaker or not, chances are good you make grammar mistakes without always realizing them. A common example of this for the English language is "there/their/they're" since they are all pronounced the same, but each mean something different.
For me it's stuff like:
-Confusing the spellings of "Rouge" (the French word for "red") and "Rogue" (like the X-Men character) due to the U and G being the only aspect that's switched around.
-Never remembering what words are exceptions in that whole "I after E except after C" thing since some words don't follow that rule
-Sometimes I put the wrong "here/hear" since like the above mentioned "there/their/they're" situation both words are pronounced the same