For a long time, the altar guild were confounded by some wine stains that would not come out of the altar cloth (almost weekly some little drop or two of the wine, pre-consecration and post, finds its way onto the spotless white altar cloth). There is, as you know, an entire ritual of cleaning that must be done to the altar linens -- I don't know anything about that end of it myself -- but finally this got so bad that they consulted professional cleaners for advice. After some investigation, it turns out that the stains are actually blood. Real blood stains, where the communion wine was spilt. This is ongoing at my parish.