Can be from pronunciation, grammar or vocabulary. Can mean using native words in place of SL ones, replacing SL phoneme with one from NL, calquing, etc. These can be due to problems in the textbook, teacher s mistakes, uncorrected mistakes made by the students, etc. Thus, a rule that the student knows to be true in some situations is overgeneralized to new situations where it does not apply. The constant emphasis on familiar forms in the language learning process is partially to blame for this.
Some errors produced by SL learners are due to the actual strategies that the learners use.
These can be such techniques as imitating others, guessing, hyperregularization, etc. This will force them to improvise. They will have to circumlocute, coin words, approximate, etc. Selinker and Lamendella distinguish between fossilization and and stabilization of interlanguage terms. Fossilization refers to permanent retention of non-native interlanguage forms.