In my experience it seems to occur when a composite brick which has been mirrored is then copied to many or several other glyphs.

@Meek: This happened a long time ago with one of the fonts after saving, maybe it broke because I had many tabs open, and that font had a lot of composite blocks? But I'm not sure. By the way, the function described by elmo also does not work quite correctly, you cannot replace composite blocks, and if you start changing the font after saving, all old blocks will go back.

It’s never happened to me, but I’d imagine that if you had one font open in multiple tabs at once, that’s bound to cause a few problems. Just an idea.

I have had this problem several times, but i have never duplicated my blocks and it also happend when there were no composites, it seems to happen after saving and reloading or spontaneously when after saving the save button turns red while you didn't do anything.

Another anoying thing is when downloading a font might still be corrupted while the fontsruction is fine, the only way to solve this in my expirence is to copy the fontstruction then see if something is wrong fix that and then download it.

It also doesn't seem to care about if you have 70 or 5 blocks altough i've never seen it happen whit just one block.

 Yesterday I had the problem again that even if I deleted bricks that were displayed incorrectly and replaced them with others, they were still displayed incorrectly. 
Once I replaced them with others; then I tried to delete them and replace them with the "right" ones; I saved everything each time and left the fontstructor to preview the font. The errors were still there. I closed the browser, reopened it, logged in again... all the same. I got the message that the server on which the page is stored is not responding... 
In the end I could only fix the error this time by "overwriting" the swapped bricks as far as possible. In the past, when this was not possible (composite) I simply started a new fontstruct.
Yesterday (as before) I tried to remove all combined bricks that I had created but not used. This way of removing bricks - in my experience - is always problematic. Some just stay there. And they usually cause the problems.

Another effect occurs when I change the order of the bricks in the palette. It can happen that the cursor arbitrarily highlights a brick in yellow and shows it, even if you change from pen to eraser, from brick to hand, to preview or even the Unicode mode. This is often the moment when the brick palette changes and some bricks are exchanged for others. 
It also happens that by using a short-cut (h/p/c/z...) to undo actions etc., restore or go back to the previous version, another glyph table opens arbitrarily. This can also cause the font structure to be modified after/while saving, logging out and logging in.

These are my observations for the moment. All this seems to be quite complex. How the duplication of some bricks comes about is still a mystery to me (12 x square bricks; 5 x triangular bricks...) If this phenomenon appears in the palette, the font in the download is mostly incorrect for use. This means that some bricks suddenly do not work anymore, are exchanged and/or twisted.

I think the more complex the structure (even if you don't use many components) the more vulnerable the code is.

??? : ))

What Beate is saying has happened to me too, step by step. She has explained it very detailed and well.

Concerning the shortcuts, that happens to me every time I use “FontStruct”. Sometimes it takes me to Arabic or Hebrew when I press: “A”, “C”, or “V” (or any of the other shortcuts). It doesn’t cause a major problem, though.

"Yesterday (as before) I tried to remove all combined bricks that I had created but not used. This way of removing bricks - in my experience - is always problematic. Some just stay there. And they usually cause the problems."

I have a theory on this issue. Lets say you create a 2×2 custom brick. 1st row left to right: Square Circle. 2nd row left to right: Circle Square. Then you delete the circles. You are left with a custom brick that looks exactly like brick number 110 (so there was no need to create it, but this is just an example). You use this custom brick somewhere.

Then you create another 2×2 custom brick. 1st row left to right: Square, Right Half Triangle (3rd brick). 2nd row left to right: Right Half Triangle (3rd brick), Square. Then you delete the triangles. This second custom brick will look exactly like the first custom brick. Then you use it somewhere. When you go to review your work, you will not be able to tell which custom brick is custom brick 1 or 2. Unless you delete all instances used of custom brick 2, there's no way to get rid of it.

For this reason, I've always wanted fontstructor to have the ability to highlight the selected brick (if 1 is selected) in the brick pallet. Rob? Would that be possible?

"Concerning the shortcuts, that happens to me every time I use “FontStruct”. Sometimes it takes me to Arabic or Hebrew when I press: “A”, “C”, or “V” (or any of the other shortcuts)."

I think this is a matter of which element has the focus. If you switch from, say, Basic Latin to More Latin and then press A for select all, the page switching element still has the focus and it switches to Arabic page. Perhaps the focus can be manually assigned to the grid after each mouse up event or something (I don't know HTML5, so guessing).

Hello.

I haven't been fontstructing lately, but these bricks swapping problem existed like always :) What I remember, I could get rid of the problematic ones using the Shift+T command, then closing fontsruction, then opening again, checking what's been deleted and not. That Shift+T command was one way to couping with these annoying brick swaps.

I've just encountered this issue. I exited the fontstuction page and found that the preview looked weird, so I entered fontstruction again and found that a lot of the bricks were swapped with others. After doing some searching I found out about brick swapping and tried to swap the wrong bricks with the right ones, but only ended up creating duplicate copies of the same brick in the My Bricks window. It's still like that and I have no idea how to get rid of the duplicates.

Thank you very much, dear bigchief!

I know, why is the bricks swapped? 

I got this problem several times before. It mostly involves me using Fontstruct on mobile devices in which the brick swap issue (and sometimes corruptions) occurs a lot more often rather than using on Desktop.

On mobile, what I usually do to get the Brick Swaps is accidentally dragging a brick in the My Bricks onto inself, the issue will select a random brick (more than 2 brick swaps can happen at once as well) that you used and replace it (on all glyths) to the brick I dragged. This type of Brick Swaps activation is kind of random and isn't really common at all but if you also happen to be a Fontstructor who frequently uses Fontstruct then it will be quite a big problem. Although, I haven't witness the Brick Swapping issue on desktop, yet.

This could be quite an issue as you can just ruin your entire Fontstruction project without creating a backup, if the Fontstruction is large and includes plenty of brick composites. To me, this made me quite tired to repair the damage everytime that happens. Gladly, the Brick Swapping issue doesn't seem to affect Fonstructions that only use block bricks, as far as I know.

like this?