Q: What is wrong with...

..."the thing is is that..."
"The reason being is because..." is one of the worst.

A: All you need is "The reason being..." and then state the reason.
eg. "I went to the store. The reason being, I needed some toothpaste."
I have heard people use "is" as many as three times in a row.
eg. "I want to go to the store. The thing is, is is that I don't have any gas in my car."