I know a lot of S's that thought they were Ns at first. It's not that they are lying to themselves, it's just that they don't understand what the S/N preference is asking. Think about it... if you described the difference between S and N, most people might want to think they are an N. But remind them what it's asking: "How do you take in information?" That, in and of itself, is kind of hard to wrap your mind around if you're unfamiliar with type preferences. Sensors might be unaware that when they brush their hands along bushes or watch your facial expressions and respond to them that they are taking in sensory information.
And just like Jaguar said, just because you're a sensor, doesn't mean you're not intuitive. Me and my ex (INTP) took the test together our first time. We both ended up as ISTPs. Then my sister thought I was an INTP. It was all confusing at first, but when I read that ISTPs are frequently mistaken as either INTPs or INTJs, and that they sometimes can relate more with NTs than SPs... it all made sense. My INTP ex likes to focus on his laid-backness and practicality, but that was something he learned from his parents (who were very strict, practical people...maybe SJs.), so he came out more S than he is N. But naturally, he's an N, with a high Si. I was an ISTP, with a high Ni. A lot of factors can influence how much you use a function, and some Ss may use their N primary function more than others. Especially IPs and EJs because their two perception functions (N and S) are their two middle primary functions. That's all the more reason why as an ISTP (Ti Se Ni Fe) and INTP (Ti Ne Si Fe) can mistake themselves as one another.
I realize you said that this is not really about your friends, but sensors who think they are intuitives... but if you find yourself really annoyed by it, just explain to them in a very matter-of-fact way what a sensor is and even let them know that just because they are a S, doesn't mean they lack all intuitive ability. As I said, my Ni is pretty high for an S, but that doesn't make me an N. It's a good rule of thumb to know what you/they are more sure of... are they more sure of what their judging preference (T/F) is over they're perceiving preference (S/N)? Then that's an indicator that their two perceiving functions are in the middle and can be confused at times, and are most likely an EJ or IP. I find it doubtful that an ISFJ (Si Fe Ti Ne) can relate to an INTJ profile at all (Ni Te Fi Se)....That's beyond disillusion. So I would try to retype him.