There ought to be a good set of FOSS unit tests for those who dare implement their own crypto. For instance, you let it hook in to your PRNG, and it'll tell you if the output is random-looking enough.
It wouldn't be a panacea for bad crypto, and it does create a risk of people thinking "oh, it passed all of the tests, it must be secure," while still implementing it overall incorrectly. But I still think it would mitigate these "foolish/easy" errors and allows devs to focus on proper overall implementation.
Or does something like this already exist?