When under an anaesthetic there may be no conscious or unconscious memory of the operation itself. Otherwise, most current physiological research seems to strongly suggest that if it happened (and physical neurological issues are absent) then you will be able to remember the event especially if traumatic.

It is also worth noting that our memory is self summarising over time. This means that we naturally delete minor details from our memory of events. This is a natural process that allows us to deal with the vast amount of information we accumulate over time.

Dissociation is a protective mechanism that is sometimes experienced to enable us to survive a traumatic experience. Broadly speaking there are two opposing schools of thought when it comes to dissociation as a cause of amnesia.

(1) Some contend that memories can become suppressed when we experience this type of protective dissociation. This probably dates back to the theories of Freud. (2) More recent theories that factor in current physiological studies suggest that although the memory may be fragmented or difficult to recall in great detail, if it happened, we remember it.

The problem with theory (1) is that if a therapist expects some memories to be suppressed then it is likely that they will look for them and encourage their patients to look for them - whether they exist or not. This approach has been cited as being a major cause of false memory syndrome.

I personally side with theories based upon solid F-MRI research and that protect against the generation of false memories.