Research done by my friend at Nanjing University indicates that while there are a lot of segmental errors in EFL learners' speech, most of these errors do not cause problems in comprehension. That is to say, EFL learners' segmental errors are tolerably erroneous.
Supersegmental errors (errors in tonality, tonicity and tone), on the other hand, can cause more serious problems. In many cases, you know, difference in these aspects is closely related to meaning. Therefore, incorrect ways of segmenting intonational units, of accentuation, and of tone setting, can result in the failure to convey the intended information.