It can be pushtx'd to you too, I think, when a peer invs it to you despite feefilter and then you fetch it. Violating feefilter was at least on prior versions not a banscore 100 event.

I'm not sure if it is still possible to send someone a 

 straight without getdata (used to be though) which will put it in your mempool but it won't be relayed.

mempool

Why is this in my mempool?

ThatWhichisNotSeen

It can be pushtx'd to you too, I think, when a peer invs it to you despite feefilter and then you fetch it. Violating feefilter was at least on prior versions not a banscore 100 event. 

I'm not sure if it is still possible to send someone a `tx` straight without getdata (used to be though) which will put it in your mempool but it won't be relayed.