Ran into a funny one today while working through learning the CraftCMS demo site:
With happylager.dev set up in MAMP on MacOS, I got spun out trying to determine how to edit the homepage content on the demo site because the admin page at http://happylager.dev/admin/entries/homepage/2-homepage looked like this:

Being new to Craft, I didn't realize it should look like:

After a while, I looked at the source and found this:
Fatal error: Maximum function nesting level of '100' reached, aborting! in /Users/max/Code/Craft-Tutorial/craft/app/models/BaseModel.php on line <i>258</i>
Since I don't really use MAMP for development anyway, I simply turned off xDebug in its PHP settings:

and now the admin side of the Craft demo is working as it should.
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My plan for this week is to get a complete functional prototype of a client site built to match wireframes. Lots of content types that Craft seems well suited for. Pretty stoked.