The background bug has existed for a long time and I think I have pinpointed what causes it. This glitch exists in both flash and HTML5 versions of flowlab.
Basically the glitch is the following, you add some objects to the background layer, then play the game and the objects in the background are solid and you can interact with them. You then can reload the page and the objects correct, reloading the page however can cause other problems such as stuff not saving. However this doesn't always happen because there is one small step in triggering the glitch that is very easily overlooked.
In order to trigger the glitch make a new game with some kind of character you can test with, then add some blocks to the background layer were you can touch them with your character, hit play. These blocks should not be solid. Now go back into the editor and open the sprite editor for the background blocks you made earlier. Enlarge them and fill in the enlarged area. Hit play again and these blocks will be solid, even though there on the background layer. Now reload the page and the blocks will resize themselves to 1 by 1 and again not be solid.
Resizing blocks causes a lot of other glitches too, mainly the resize simply not saving if you reload the page soon after. Along with some other really strange stuff I ran into when making Gravity Hop.@grazer
can you please look into the background glitch along with some of the other resize glitches, I run into them often and always just have to find my way around them, it can be very annoying sometimes.