I'm betting it comes down to literacy rather than fluency.

For example, one that tends to trip up a lot of people is the contraction of "would have", which is commonly pronounced "wood of", and frequently spelled "would of", but is more properly spelled "would've".

I suspect this is the case for the submitter. "Once in a while" might be spoken as "once 'n' while" by somebody who tends toward rapid speaking (that tendency we all have to some extent to blur our words together when speaking). Submitter may even just say it that way out of rote habit.

It could be the submitter was attempting to write something spoken without knowing the correct (look at me getting all prescriptivist) way to spell it.

Then again, it might just be a typo.