Reading some litrature on these things, it always seems to be the simplest things that snowball. Not knowing how much fuel you have is simply just forgetting to check. There are probably a host of reasons as to why the check(s) didn't take place.
One potentially lethal but essential characteristic of skill is that when you learn to do something well, it is often to the extent that you don't need to think about it anymore. For example landing - most of you probably don't think how to land you just do it. At the more procedural level, you would be a dangerous pilot if you had to think through everystep to complete it, you would not have the capacity remaining to complete other tasks such as listening and looking out. I think that, as you gain experience, you do more and more automatically, subconsiously and as part of the procedure and routine. The disadvantage of this is that if you get distracted then you may subconciously miss a step, or apply the right action to the wrong thing. In addition, if you do something routinely and very frequently you may well start to not use the checklist. You may have it in your hand but not follow it; I found myself doing this once or twice.
Personally, I think it is frighteningly easy to screw up in a major way. I am also convinced that believing this reduces the chance of screwing up.
The biggest problem with stuff is that we can't choose the mistakes that we make, we can only attempt to make the most dangerous ones the least likely.