Actually rug I'm pretty sure that was me and I remember it very clearly, sorry to call you out on it but I'm glad you bring it up because it is a good example of the subtleties of how the rule works. You had it a bit backwards... It was my unit that your revenant was shooting at, with your other revenant and a hill (building?) in the way. Revenants can't be inside other revenants obviously and hence the relevant rule is the part about intervening terrain (one was effectively intervening terrain for the other).
What was at the root of the confusion was where you measure LoS from. You were concentrating only on the revenant, which it was perfectly possible to draw a line past. Somehow you inferred that I was saying you had to have all parts of the target visible to all parts of the shooting model (i.e. totally unobstructed). But that wasn't it at all. So long as you can draw a line from any part of your revenant past the other revenant and see all of the target model, there would be no modifier. (Some like to measure from specific points or the centre but I prefer to do it however my opponent likes when it comes up - or to put in your words, when they "suddenly remember" it's supposed to be played in whatever way works best for them at that moment...). So in actual fact your problem was simply that you had positioned it so that the sight lines past the revenant were partially obscured by the hill. Or to put another way, there was no line that could be drawn from the right hand side of my unit to any part of your unit - all lines that cleared the hill were blocked by the revenant, and vice versa. All you had to do was move slightly differently and it wouldn't be a problem but you went into "Ugh I have to explain the rules" mode so it took a little longer to get there.
I might not be the most competitive player or care about winning but IMO I am pretty good about parsing the rules and not playing for advantage so maybe next time you'll give me the benefit of the doubt
Again, sorry for the "erm, actually!" routine on here - it was no biggie on my part and in hindsight I probably should have just ignored it but I thought it was a simple mistake on your part in a clear cut situation and I always give people the chance to change their mind. (And yes that means ignoring the "touch move" rule from 1.7). My army was never going to win any of its games so I was pretty relaxed about everything really!
Speaking of ignored rules, I know what you mean about "communication failure", I notice it too and it's soooooo common for people to omit the action. It really doesn't bother me unless they actually fail to nominate the formation, but even then I tend to just mention it and let them get on with it. Usually it's obvious which one they intended to activate anyway, and declaring the action rarely has any consequence on the game except to reveal their failed plans. In some ways it'd be better if the rules said to roll before stating the action anyway.
But in the case of infantry, we measure from any of the models on the stand, instead of just the base (and our WE and vehicles have no bases so from the models too).