Focusing On An Input Box Quickly Will Delete The First Two Characters Typed!
I figured out the answer by myself.

Input boxes in web forms tend to lag a little, and so does my selection. If I select all the text in the input box, dragging my cursor from the end to the beginning, the selected area will lag behind half a second.

That lag is what causes the problem. When I start typing afterwards, the selection is still active, causing it to replace one or two characters if I type too fast.