Negative R-Squared

I am reviewing a few articles and come across a low value of R-Squared. Did some research and come across the following explanations. On negative R-Squared:

https://stats.stackexchange.com/questions/12900/when-is-r-squared-negative

Mathematically possible if you constraint intercept and slope, but maybe "pathological."

On Influence and Leverage:

https://rpubs.com/rachitkinger/intro-to-diagnostics-multiple-regression

Here is the Q&A on the issue of low R-Squared. 

https://stats.stackexchange.com/questions/12900/when-is-r-squared-negative