But lets look at it from the the other side. You see a 10 BTC sell order in a self-regulated exchange for $100 per BTC and you buy them. Would you feel good about it ? Would you consider this behaviour ethical ? If you don't buy them, then another user will buy them. Would you hesitate to buy them to not have any compunction ?
If it was me, I would feel good because I got a huge discount and I don't see any wrong on that action. In the sense of being ethical, I believe it is because the buyer bought it legally. This case is almost the same as seeing money on the street and picking it up not knowing who lost it. The only difference is that we know that the seller intends to sell that BTC, whether there is a price error or not is out of my concern, I do not forced or trick him to sell at such low price so why would I feel that I wronged him.
Does it make any difference, if you actively buy them or just had a order in the market that mysteriously got filled ?
It does in a sense of awareness.