Let $G$ be a graph. I've heard that, if we use nauty to canonically label $G$ on two different platforms, it's possible to obtain distinct labels. However, I've never actually seen this occur.
The nauty user guide (pdf) writes:
Beginning at version 2.1, the canonical labelling does not depend on the compiler, the system, or the word size.
This seems to imply that it's still possible to receive distinct canonical labels on different platforms. (This might also be somewhat out of context, since this sentence is written in the "dreadnaut" section.)
Question: What is an example of a situation where, on two different platforms, nauty would canonically label the same graph differently (if there are any)?
If it can still occur, is it a rare occurrence?