I just had a Masyu puzzle that was much harder than usual in its category. Maybe I missed something obvious? I had to use a lot more of my bag of tricks than I usually do -- my completion time was 9:08.47, and I normally complete these by about 5:00.
If you want to give it a shot, go to this page and make the following selections:
- Size: 25x25 Normal Masyu
- ID: 3,787,521