Often times, when I place a torch, things go okay; the torch places, and it lights up the cave, my house, etc. However, let's say I went caving and ran out of torches, so I decide to double back, grab all the ores I saw, leave the cave, and decide to come back in a few (Minecraft) days. Then, lets say I need to get off my computer because it is lunch time, so I save and quit my game, and then get back on. I decide it's time to go back to my cave. I go back to my cave, and some of my torches are falsely depicting light levels. A torch might be placed, but it only lights up blocks on the left side of it or the right side of it, or it might not give off any light at all. I have not figured out the algorithm behind it, how it happens, why it happens, or what percent of torches are affected. The easiest fix I know of is to break the torch and put it back.

To find the issue:

     -My best guess would be to go caving, then leave the world, rejoin the world, and then
     check your torches. (Note: this also affects torches I placed human weeks ago.)

To solve the issue:

     -Break the torch causing the problem and replace it.

     -If issue is not of torch light, break and/or place a block in the affected area.

Other things to note:

     -This happens to any source of light, not just torches.

     -A lot of times, this happens if there is an obstruction to the light (e.g. the shadow of a tree, 
     the shadow of an overhanging cliff, etc., but mostly in corners of caves)

     -The above is not always true.

     -I have not figured out if this bug affects mob spawning, so it may, but it may not.

     -I do not have any screen shots, but if this happens again in the near future, I will get a     
     screen shot and post it in the comments.

Thank you for looking into this issue and possibly solving it. Enjoy your week!