The issue is, that when you start typing a word and then move the cursor to the beginning or middle of the word to refine your query, then the cursor will jump to the end as you are typing. In short, Google is moving your letters from where you want them to go to another position.
I have not been able to replicate it but here are a couple explanations from frustrated searchers:
Search "testing" Results show up Go back to search box, type more text in the middle of the word. After a few seconds, the cursor moves to the end, and any text you type will be appended to the end of the word, not the middle where I put the accursed cursor :(
This is realllllly annoying because I do a lot of searches, and often refine or modify them quickly. This only started happening today or yesterday, is there a fix?
I searched for "hello", the results page loaded, then I put the cursor in the middle of the word and started to type "testing". But after "testi", the cursor decided to move to the end, and the "ng" part of the word is misplaced.
This user shared a pretty bad screen shot over over here but what I'd love to see is this happening in real time, a screen cast.
This may or may not be related to the Google cursor box not focusing issue from earlier this week.