In error messages, if a block is nested inside another block of the same category, colors do not alternate:
The bug does not happen in the same session. To recreate, create a custom block, one that will trigger an error message, like:
If you bring up the error message right now, the alternate nested colors of the blocks will be correct.
However, save the project. Reload the page, and open up the project you just saved. The blocks in the error message do not alternate colors.