The best kept secret traffic rule, in my opinion, has to do with the obligation to have your headlights on, regardless of time of day, in certain circumstances.
The Ley de Trafico, article 42 on the obligation of use of lights during daytime hours, apart from the obvious, re tunnels and bad weather conditions, mentions the following.
If you are driving in a lane that goes in contrary direction than usual, or in a lane that can be used for both directions, as is often the case when roadworks occur and you are lead onto a lane of the contrary direction, as the usual one is blocked for maintenance, then you are obliged to have your headlights on.
As these temporary lanes are usually indicated by yellow/orange painted lines on the road the obligation to have the headlights on is often associated with the presence of these yellow/orange lines.
Beware though, as a reversible lane is very common, even without roadworks present, on many 3 lane roads where the lines are alternately painted solid in one direction and then the other… to alert the opposite traffic to the fact that you are making use of the lane you are supposed to have your headlights on there as well, and not a yellow/orange line in sight there.
I have researched the guidelines for driving instructors and not even there is any mention of this rule, but we have received notice that an expat has been fined for non-compliance only recently, so be warned and please share this information with friends and family.