Sounds more towards the back of the mouth are often more in error. Sounds that are grouped, like K and L together, are harder than consonances singly. Consonants are often harder for a speaker than vowels, but we do get vowel or diphthong dysfunction and interestingly, that vowel distortion can have a very adverse affect on intelligibility. Sounds at the beginnings of words are often harder for speakers. Sounds that occur less often are harder.
Errors increase as word lengths and complexities increase.
Sounds are often harder when speech is what we call purposeful, as opposed to more automatic or reactive. Speakers often have difficulty imitating sounds, as well as producing them spontaneously. One of the really frustrating things for speakers is that they are aware of their errors. So they try, often, to make corrections, but these corrections are usually as mispronounced as the initial sound or words were. It differs, as I mentioned.