What student pronunciation errors do many native speaker teachers have difficulty in correcting? 

Here an entrance about English pronunciation appreciation.
Pedro Mendez: What teacherrs have to be worried about is teaching students respect standard English pronunciation. For example if  they teach british English, they should use BBC standards.

Tony PenstonOwner, TP Publications Native speaker says that teachers often do not have a detailed knowledge of phonetics. Of course they can correct many mispronunciations in the 'listen and repeat' way, but I'm sure there are errors that need more than this. If you have experience or knowledge of areas needing phonetic skills I'd appreciate hearing from you.

Blaze Blaze Miskulin Teaching Supervisor at Cicely Language Center, claims that he runs into this frequently. There are several sounds used in English that do not exist in the Chinese language(s). The most prominent of these are the "th" sounds. For these he makes a point of showing how his tongue sits against my teeth. It's all "look at me" instead of "listen to me". For other sounds--such as the "deep R"; "are", "air", "or"--I've learned to use Chinese words that approximate the sound (十 or 日) and telling them how to modify it ("make it with your throat, not the front of your mouth). It's not a perfect way, but it's easy to understand and fairly successful. The other thing I do is, quite honestly, not worry too much about precise pronunciation until I'm dealing with advanced students. 99.44% of native speakers don't use proper pronunciation--and the few that do argue over which version of "proper" is correct.

 Liliana Liliana Di Giovanni --Freelance Translator and Teacher It is essential for any native speaker teacher to know about phonetics, although you may not teach it as a subject, it must be applied when correcting the pronunciation errors. It is a fundamental tool to know how to deal with this problem Para el blog