Australia 36c Error Or Not?

I just found this stamp in my collection. This blueish colour is an error or not? The back side of the stamp is normal...

Opinion:
Not an error. Common to see shades on stamps originating from coloured envelopes.

I have seen that "honeycombed" colour, on other stamps, its rather strange, I have not yet identified from where it originates. Certainly not from the original printings.

Printing errors on used stamps are problematic due to their additional postal processing and (oftentimes) soaking.

My initial impression of this stamp: contact with a mildly water-soluble security printing from another envelope clipping during the soaking process.

Am of the belief that it is a PO security cancel of one sort or another.

They do appear when I am sorting but 95% of them come clear to see only after soaking.

They also come in different colours and are positioned as other cancels take place only on the face side of the stamp.

Interesting, thanks John.

Decades ago, I had this problem (the patterned staining) as a beginning collector. The lining patterns from the inside of the envelope always transferred to the front of the stamp underneath it in the soaking bowl. So, I became more careful with sorting out the stamps with the blue security patterns on clippings.

Then, the only soaking stains were those on the backs of stamps.."place stamp here", etc.

Bob...I think you have nailed it !