Consider the ways in which a NIL can occur and you'll understand the difficulty avoiding them:
- The QSO was logged with your call incorrect and the log checker software failed to match the erroneous record with that in your log. Software isn't perfect so this will happen. There is little you can do about it other than to be certain the other station has your call correct.
- You are running on the same frequency as someone else who you cannot hear and you think you have worked someone who instead worked the other station. Yes, this really happens and you may not notice what's going on for several minutes. All you can do is QSY and hope for the best.
- User error results in your QSO not being entered into the other station's log. Every contester makes mistakes so this will happen occasionally. I know I've done it myself and unless caught instantly there is no recovery possible, and indeed you may be unaware of the mistake.
- The other station gives up on you because you're too difficult to copy -- happens a lot when you run QRP -- but instead of telling you sends "TU XX9YYY" and continues onward. You think the QSO is good and log it. In my opinion this is unsportsmanlike behaviour.
I was surprised to find that I could remember a few of those NILs in this most recent LCR. A couple of them I was sure were good. But as noted above the log checker may have gotten it wrong. Although I didn't bother this time a couple of years ago the CQ WW contest director at the time suggested looking in the public logs for these NILs. It was both enlightening and perplexing. By comparing logs you can sometimes see where the software may have mismatched records but you can not see into the other operator's mind if your QSO is absent.
Learn what you can, do your best to ensure your future logs are accurate and hope for the best. Since many of your competitors are seeing similar penalties the score reduction is only a problem if you are especially negligent, in which case their lower error rate will hurt you. Perfection may be impossible but you will surely not attain it if you don't try. Accurate logging is a valuable skill for contesters to practice. I can do better.
But what if you're not a contester? Are you a DXer? The possibility of NIL still applies to you. Let's examine this with a real situation I encountered, but without revealing call signs.
A few weeks ago there was a DXpedition from a moderately rare country, one I've worked many times before. When they showed up on 40 meter CW I jumped in if only to practice my pile up skills.
After just one minute I got through -- having a yagi up high helps! The band was noisy here and presumably there as well (warm weather in both places) with the usual QRM from poor operators who keep calling regardless of the station being answered. As a result he took a few tries to correctly copy my call. Then without having ever sent my correct call (one letter was wrong) he sent "TU UP" and moved on to the next QSO.
I may have been in their log with my call correct, my call incorrect or perhaps he gave up and erased the QSO. There was no way for me to be sure. What would you do in this case? In a contest I would most likely have logged the QSO, risking a NIL, and perhaps duped him later if it was a needed multiplier.
However this was not a contest. If you are a DXer you'll have been in this situation many times and faced the question of whether to log the QSO. From my experience I know that many would log the QSO and either hope for the best or check the DXpedition's online log and try again if the QSO doesn't appear.
What would you do? Be honest. Think about it for a moment before you continue reading to discover what I did.
I erased the QSO from my log. To me that was the ethical choice since I did not hear him send my call correctly. In effect I assumed NIL or copying error, making the QSO invalid. Had this been a new country or band-country I admit I would have been tempted to log it and hope. When I hit "delete" I was motivated by the disgust I felt at the poor operating practice that left me in doubt.
A day or two later I was talking to a friend who congratulated me on working this station on 40 meters -- we tend to check up on each other in DXpedition online logs so that we know when to call each other when we hear them on. I had to tell him that, no, I didn't log the QSO and proceeded to explain why. He had a good chuckle over that one.
All of this leaves us with an ethical viewpoint on the NIL problem. During contests we expect to hear our call correctly before logging the QSO, although some don't care because it's the other guy's score that will suffer, not theirs. Indeed when you're running it is common for callers to never send your call so you can never know if you're in their log or logged incorrectly. Some always send the running station's call to remove doubt, which annoys some operators due to the two seconds it consumes.
For DXing both stations do try to ensure correct copying of our call since the penalty for a NIL is arguably greater for the rare ones. We all notice the sloppy DXpedition operators who do not strive for accuracy, leaving us in doubt and annoyed. We also notice the sloppy callers who pay little attention to whether their call is being correctly sent in their enthusiasm to work the DX.
In DXing as in contesting it pays to be accurate and to confirm or correct what has been copied.