Breakdown is an unexpected event within a system. If a user is told that if they carry out action A that either result B or result C will occur, and that nothing else will happen, then any other result is a breakdown in the system.
Examples occur frequently in computing .. and they occur at different levels. An interface may breakdown when function keys that the user is not aware of are inadvertantly hit. The user is not aware that they have requested anything and yet something quite dramatic may occur.
Essentially in any use of any system, the user builds a model within which to interpret the system behaviour. When any event occurs that cannot be explained in the existing model, the system has broken down and the user must build a new model, one that includes the new event.
It is the system designers role to minimise such breakdown, to provide a ready-to-hand environment that introduces the user to the concepts needed to interpret any perceived behaviour.
It is for this reason that naive users should be sought for testing, and that their initial approaches and reactions to the system should be analysed carefully.