Some problem consonants and consonant contrasts of English
We can identify 'problem sounds' by three main ways: (1) differences across English and the L1 of the learners (contrastive analysis), (2) the actual pronunciation errors L2 learners produce (in general or based on language background), and (3) 'marked' sounds of spoken English that learners of various backgrounds might find difficult (for example, they are late in native acquisition processes, or they are rare sounds among most world languages or at least some major languages). Here is a set of problem consonant sounds and contrasts that I have compiled with all three above in mind.