Why does Podcasts play an un-downloaded episode?

Basically I've got lots of subscriptions to various series of podcasts, but not all topics interest me, so I manually download the ones I want. This means that there are still episodes appearing in the list with the download arrow next to them, but I havent downloaded them at home over wifi (to save my precious data allowance). A couple of times if I havent been paying attention when out and about, the next podcast begins to play - even if this is one I havent downloaded, so I can only assume that if im not at home this will use my data (and get me listening to a podcast i didnt really want to as well!). Is there a way to stop this from happening - i dont want to delete the episodes from the list in case i change my mind later and do fancy listening, but similarly, i dont want to get halfway through the month and realise i've used all my data.

Thanks,

(ps - autodelete seems to be random on my phone, as when i view podcasts on my phone via itunes, some have been deleted and others not!).