If my students make mistakes I try to observe what type of mistake it is to see how can we fix it.
Students don’t like the teacher to correct them most of the times; so I wait to see if an insight self correction happens during the lesson; or if this mistakes happens several times or to several students I explain the grammar and examples again or if it’s a fossilised error I talk to the student him/herselfabout it right after class.
A very common interference error in pronunciation is adding a strong E in fornt of words like :stereo, statue,etc. So I try to emphasize in general about this error once it starts happening, during class.
We really need to bring the pshychologist or well observer from us into class so we find out which type of students deal better with correction; keep in count their stages of learning to see if it¿s a developmental error and therefore will be fixed along his/her next step in the stage, also we need to prepare and know our lesson well iwth different activities in hand, so we can detect if an error is really that important to clear up in class or not.
Even with the teacher explanations supported with vocabulary, conferences and posters on the walls, even with repetition of these mistakes several times to the group and individually, students keep making the typical third person “s” mistake; throught the years experience have thought me it’s important to point out as much as needed (until the insight happens and they stop making this mistake); and to on purpose have activities where they will need to avoid the “s”, because it’s exactly from this errors and self evaluations how this problem can be solved.