Alwyn Cosgrove has an interesting post on "getting out of your own little world". He's primarily talking about business, and fitness as an aside, but the idea applies to martial arts, combat sport, and self-defense as well.
People tend to seek out information that reinforces their beliefs. I am guilty of that as anyone else; I like Rory Miller's work for a variety of reasons, but one of those reasons is that it lines up well with Tony Blauer's work. Is there a 100% match? No, but the framework that underpins both is close enough that I can fit them together in a way that reinforces my goals. That's a good thing, in many ways, but it is also dangerous.
If all you ever do is train things that reinforce your beliefs, you start to drift into weird territory. One of the strengths of combat sports (and part of why they are so appealing) is that they have a built in system for challenging your training. If you think that you've discovered a secret technique that makes you completely immovable, go sign up for a Judo tournament: you'll figure out how movable you are or aren't real quick.
The danger, of course, is that all systems, even combat sports, are ultimately self-reinforcing. Boxing works great when all you can do is punch, but when you put a Boxer in a Muay Thai or MMA fight (without any cross-training), they have problems. Sport Brazilian Jujitsu players use techniques that would make no sense as soon as you introduce striking. The tall, upright stance of some Nak Muay becomes problematic when takedowns are allowed. And so on.
Martial arts can be even worse about this, creating bizarre, incestuous little worlds where people develop counters to techniques that no one else uses, or find ways to make themselves invulnerable against people who already believe they are invulnerable. The worst end of this turns into martial hucksterism, where teachers claim to have powers and abilities that Yoda would find implausible.
Self-defense systems can drift that way too. People make up weird ideas about real assaults, and then train for them. Or, worse, start training to deal with attackers using their system instead of deal with actual violent assaults...and then things get bad.
How do you avoid this?
Go outside your paradigm. Sometimes a little outside. Sometimes WAY outside.
A little outside might be reading, or experiencing, some slightly different training. How different depends on your goals. If you're a boxer, and want to box, you probably don't need to go do Muay Thai...but you might examine how the Thai punch, and see if there is anything useful there. Or, more properly, watch some different coaches. If you play a stick-and-move kind of game, try being a brawler for a while. If you're a brawler, stick-and-move. If you're a Judo player, examine some BJJ material to get some different ideas about groundwork.
If you're a self-defense guy, examine someone else's information. I will occasionally go and read articles or forums of instructors who I know I don't like, who strongly disagree with Tony and Rory on a number of points. Why? Because it makes me think. It takes me out of my comfort zone, and makes me examine my beliefs. Even if I hold onto those beliefs in the end, at least I know (and can usually better articulate) why I have those beliefs.
Going WAY outside is different. Cosgrove uses the example of looking at Starbucks and Disney for business ideas about fitness. Go to something that is far enough away that it is untainted by the inanities of martial stuff. (It will be tainted by its own inanities, but that's okay.) Read about how chess players strategize, or how football players learn new plays. Go some place that's close enough that it has application, but far enough that it forces a different line of thinking.
My own personal example on this one: my wife and I take ballroom dance lessons. At its core, dance is about knowing how to feel, read how another person is moving, and guide that movement. All skills critical for fighting, but in a completely different way.
Obviously, you can't do this all the time, but it's worth doing occasionally.