> sometimes do
> rarely do
Are terrible for maintainability. Imagine that you are reading through the code of a programmer who used this library - What is 'sometimes'? What is 'rarely'? It would need to be documented on each usage to save half an hour of digging and cursing for the poor sap who encounters it.
> 1.percent_of_the_time do
For the same reasons, this would be better named "times_out_of_one_hundred" since The Time usually means a time of some sort being somewhere (i.e. the 21:43:21 type of time).
At least you foresaw some peoples' reactions to this:
https://github.com/sudara/sometimes/blob/master/test/helper....