Incidents within tunnels, whether involving fire or not, may be regarded as falling into either of two categories: (1) those incidents which may be regarded as having been explicitly anticipated as possibilities and (2) those incidents which may be regarded as not having been explicitly anticipated, i.e. those incidents which may be regarded as having been unanticipated. This classification would also apply to safety-related incidents whether they involve fire or not. It may be the case that an unanticipated incident may have a significant adverse consequence. Also, an unanticipated sequence which does not have a significant adverse effect may still be very important in that one or more causal factors may exist also in a different unanticipated sequence which does have serious adverse consequences; the effect of a causal factor depends on context. (This paper does not address the issue of what may be regarded as ‘acceptable’ or otherwise.) Here ‘consequence’ is taken to relate to life safety, property damage or disruption of operation. Given the fore-going, it becomes of primary importance to consider why an incident may be unanticipated and what might have been done within the system, before-hand, to mitigate any adverse consequences of an unanticipated event. This paper attempts to explore these difficult matters and aims to bring to the surface some pertinent issues and factors so they may be discussed further and possible measures taken with the ultimate objective of trying to ‘allow for the unanticipated’.