There is no objective definition of a “speech error”, but there are some guidelines you can follow to decide whether you have found an error. Every person has their own dialect, an idiolect. There are normal pronunciations of each word in each idiolect, and there are normal ways in which those pronunciations are affected by context and rate of speech. When a pronunciation is normal taking these factors into account, it is NOT an error. In other words, errors are not defined relative to a standard dialect (or your dialect). In many cases you will not know what the normal pronunciations are in the idiolect of a given speaker, but you can make educated guesses.