Sometimes when a sign is unloaded and reloaded, it gains the oak wood planks texture, and WAILA says it was loaded incorrectly.

I have only noticed this with iron signs (as those are what I usually use).

I can get a screenshot, but nothing shows up in any logs (did a lot of grepping), so I don't know what could be causing this.

The game doesn't crash, it just doesn't display the signs correctly. Not all signs do this, strangely.

A tip to help debugging
i dont know if you have access to the save file
but load it in to MCEdit, and send a detailed structure of the NBT (if you hover over it, you will get a NBT structure of that block) by this way the develper can debug a lot more what could be happend

I just checked the NBT of that sign and one that worked, and there were no differences (I had created both signs with the same text). This issue appears to be in the code that loads the NBT data for the sign.

I have no idea how to export the NBT in MCEdit2, as that is what I am using (MCEdit1 doesn't load the worlds, and crashes a lot for me).

The block shows up as a ? since MCEdit doesn't know what it is.