I have several outside motion lights (infrared detectors, I think).

So anyway, one of them just decided to stay lit all the time.  Did the electronics inside just go on the fritz?  This is the third time this has happened to me.  One time it was a detector right out of the box that would never work properly.

The simple things I know you will ask:

It's not from a heat source.
It's not from movement.
It has worked fine for a long time (and so did the other one).  Only one out of the box gave me this problem

Is this a common thing or is it just me?

only 24 reads?  someone here has to know

It's happened to me.  You can replace the sensor unit on some but it's a pain in the ass and for all that grief you may as well go new. Maybe change brands next time.  Currently using Heath Zenith (Home Depot).  4 years on one, 6 years on the other.

Maybe you have a chupacabra by your house and he likes that particular spot?

The electronics inside are the absolute bare minimum cheapest crap they can get away with, to remain price competitive.  Not worth fixing even if you could somehow find the parts and someone who knows how to troubleshoot electronic components.

I've been an electronic tech since 1964, and when cheap crap like this dies I salvage the bulbs or whatever and replace it with new.

Buy a good motion sensor and you won't have this problem.  The absolute worst ones that I have seen/installed (damn homeowner supplied!!!) are the Lowes ones, they are like fricken lego's!  The coopers come in a close second though.  So yep, buy a new, quality one.  It's normal for crappy motions to go on the fritz.  RAB makes an okay product for the price, and remember, it's just as cheap to buy the entire setup over just the motion.  Look at their mid-range products, in the $80 range, or even better, spend the extra $30 and get one of their better setups (they are only about $110, and again, that is for the motion and two quality heads).  

Ignore anyone who tries to tell you your lights are being triggered by zombies.  Everyone knows that's ridiculous because zombies have no body heat.  They're obviously being triggered by skinwalkers.

Quoted: one of them just decided to stay lit all the time.

Some models have a "sensor bypass" feature that turns the floodlights on all the time. Usually, it's activated by turning off the AC power to the unit for just a fraction of a second. Occasionally, it will enter this mode on its own, due to a momentary drop in AC voltage (such as when the air conditioner or some other high-current appliance kicks on, momentary power surge during a thunderstorm, etc.). Once it gets into this mode, the only way to get out of it is by manually turning off the AC power again, usually for at least several seconds.

Quoted: Ignore anyone who tries to tell you your lights are being triggered by zombies.  Everyone knows that's ridiculous because zombies have no body heat.  They're obviously being triggered by skinwalkers.

But they're motion sensors, not infrared sensors.  Zombies are still a perfectly rational explanation.

To be sure, I think you'd need a groaning sensor.  I do not believe that skinwalkers groan.