Full Idea
In internalist theories the grounds for justification are accessible to the believer, and Gettier problems arise when there is nothing wrong with the internally accessible aspects of the situation, but there is a mishap inaccessible to the believer.
Gist of Idea
For internalists Gettier situations are where internally it is fine, but there is an external mishap
Source
Linda Trinkaus Zagzebski (Virtues of the Mind [1996], III 3.1)
Book Reference
Zagzebski,Linda: 'Virtues of the Mind' [CUP 1996], p.283
A Reaction
I'm sure we could construct an internal mishap which the believer was unaware of, such as two confusions of the meanings of words cancelling one another out.