You have probably noticed students who make fossilized errors without realising what the concept is. It refers to an error that is made so often that it has become a natural part of the person's speech.
There are a number of factors that contribute to the phenomenon. One is over-generalisation of rules; for example the addition of "s" in the third person singular present. It is easy for students to remember I come, we come, they come. Students can forget to add the "s" to "he comes" because they over-generalise the rule and apply it to all the verb forms.
Another factor is the constant use of false friends. For example, in French "sympathique" means nice when referring to a person. In English, the word "sympathetic" has a different meaning. If French students are not corrected in their use of the word sympathetic, they can believe they are using it correctly. Regular use of the word in the wrong context without correction leads to a fossilized error. This concept can also be seen in the use of the present perfect. Many languages have a tense that resembles the form of the present perfect, i.e. has / have and the past participle. However, most languages use this tense in different ways to English.
The way to deal with fossilised errors is to raise the students' awareness of the error and to re-teach the language point. The teacher also needs to be aware of the error. If teachers are immersed in the culture of the country they are in, they are less likely to notice the error. There are so many things to think about when teaching and the error seems so natural, it is easy to let it slip by. When students are aware of the error and the teacher has checked they understand the language point, correction is needed each time the error is made. It can take a long time to redress a fossilized error; so patience and good-humour are needed. Of course, the easiest way to deal with them is by prevention. Try to correct the error before it becomes entrenched.