1. I highlight some text by selecting vjjj
2. I copy the text by pressing y
3. I move my cursor to someplace else in the file
4. I hit cw to change a word and type some text
5. The text that was highlighted in step 1 gets replaced with the first letter I type and the word I am intending to change gets replaced by the remainder of the text.

Fortunately undo undoes this problem but it took me three or four times of doing this before I realized what was going on.  Now, when I copy some text by highlighting it first, I have to remember to hit my escape key to "unhighlight" it before I do anything else.