Query

[rajan]

They informed me that last time such thing occurred was sometime back in 2003.

Can anybody suggest why was is used here.

[Arady]

They informed me that last time such thing occurred was sometime back in 2003.

Can anybody suggest why was is used here.

maybe because it belong to thing that occurred years back and not to the action of informing you

[Tdol]

If you're wondering why it's not the past perfect, it could be, using a standard backshift for reported speech, but we often don't bother when it's not that important.

[rajan]

If you're wondering why it's not the past perfect, it could be, using a standard backshift for reported speech, but we often don't bother when it's not that important.

Still I could not understand. If we remove "WAS", why the sentence is wrong then. How we would know that "was" is to be used here.

[Tdol]

We need it to go with 'the last time'- 'occured' goes with 'such a thing'. Without the verb 'was', the sentence would be incomplete:
the last time such a thing occurred back in 2003... (it needs more, like 'the last time such a thing occurred back in 2003, we had a major crisis.)