I stumbled onto a US Congress representative’s website with what I think is a blatant and very visible mistake:

Namely, the sentence in yellow, “How does the population change effect our district”. I am surprised to see this error in such a high-profile and (probably) thoroughly checked webpage. So, I wonder: how common is this exact error (effect vs. affect)? Have the usage of the two really started to merge, so that we should consider them as equivalent?
PS: yes, I know the dictionary says they have different meaning; I'm asking about usage and the evolution of the language.