If you click the Replace button fast enough, it skips over text that it should be replaced. You don't have to click Replace very fast for this to happen. I find this behavior highly unexpected.
Steps to reproduce problem:
1. Start a new document of about 50 lines where each line consists of a single character, "a".
2. Set up a Find and Replace where "a" gets replaced with "bbbbbbbbbbb".
3. Turn of Wrap Search.
4. Click the Replace button rapidly.
5. Many of the a's will be skipped.
If you click Replace slowly, the problem doesn't occur.