Forge 1448
FloodLights-1.7.10-1.0.1-110.jar

Perhaps an item (only available via Creative tab) that can be used to actually see the light "entities" to track down possible mod conflicts or lightning conflicts

See screenshot for example

That was taken after the floodlight (electric) was switched off.

Which mode was the light on?

/edit: Can you try this build here: https://www.ironhide.de/jenkins/job/FloodLights/114/
Turn on the light and turn it off again and see if that fixes the light block.

I will when its not so late for me (I'm UTC+10 - 10.30pm )

And i tried both cone modes (narrow and wide) - both have similar effects in different areas where there was no floodlight in the area (i moved 30 chunks away for each one)

The only way i can "destroy" those rogue light sources is placing blocks down until the block placement removes/destroys the light "entity"

But I will say that world save was from a version FloodLights-1.7.10-0.6.8-96.jar -> FloodLights-1.7.10-1.0.1-110.jar

Should not happen, I reset the blocks internal name from that version so all lights should get destroyed. In the newer version I added in a check if setting the block to air failed and if it does the light should update itself until it is able to do that.

/edit: Updated the build once again up in the comment above, try to see if this fixes the problem you have. In version 1.1.0 I will add in that debug item anyway however.

The newer build (114) works like a charm.

For reference the glass blocks were AE2's Quartz Glass blocks that the light "cones" were shining through. I had to restroy and replace just one of them - perhaps possible mod block conflict?

Edit: Sigh... Narrow Cone is fine - Wide cone has the same issues as before. And also while in wide cone, if using a system that activates based on detection (redstone - for example Draconic Evolutions Player sensor) thats when its very visible.

Hm what is your issue exactly?
One weird thing is that I have the debug tool here in my dev environment and it looks as if this bugged light blocks are not actually the tile entities anymore and they do disappear when reloading the world/restarting the server.

This is most likely a client side desync. I have no idea where it comes from though.

For what it's worth, I was just coming back to Minecraft and tried 1.7.10-1.0.2-43 and had problems with illumination not completely updating, too (phantom light that got left behind when switching modes). I tried in single player, both with and without FastCraft...

The last version I'd tried before was way back at 1.7.10-0.3.7-51 and it doesn't have this problem. So, something that was changed between these versions may be the cause?

Forge 10.13.4.1448 and Java 7-64

Weird thing I figured out. In my dev env it bugs too, though apparently the blocks lighting up are air. Seems to be a client desync as rejoining the world / rejoining the server seems to fix it.

Still have the code for the client side phantom light debug tool. Works (but a bit buggy still) and shows the entities. Gonna leave this issue open for a later build.