10 year LR application question

[immigrator]

Hello, I am considering of applying 10 years LR soon.

I qualify in every criteria. However, I had a 3 months plus 15 days on gap in my visa in year 2009, from Jan to Apr.
I was studying back then, my visa expired on Jan, But I forgot to extend it until 15th of April. Then I applied and was given a 4 years visa extension without questions. I was fully eligible to apply for the extension in Jan, but I just forgot to apply it.

I have been told mixed results, some say 75% of success, some say 100%, and some say absolutely no chance at all.

The problem is bothering me to death, could anyone have experiences and expertise give me some pointers?