I ran across a glitch right after a fresh install of version 5.4.0.5 to a 64 bit Linux server, but I can't seem to get it to repeat. I'm still getting my feet wet, but maybe this will help the developers:

The install went smoothly (no warnings). Here's the sequence that produced the glitch:
* logged in as admin
* appended a paragraph to the "welcome to concrete" block
* switched to html mode
* clicked 'update' to switch back to wysiwyg
* clicked 'update' to save my changes to the block.

Instead of "The block has been saved successfully" I got the following JSON string: {"aID":"11","arHandle":"Main","cID":"1","error":false,"bID":"34"}