Generating pending image transforms stuck
I was having this issue (w/ PHP 5.6, Craft 2.6.x, on WebFaction hosting) because it seemed like PHP had no memory limit and single processes were handling huge image transform queues, bombing when it got over 1.5gb or so.

Making sure Imagick has a memory cap seems to do the trick to avoid the queue getting stuck as PHP runs out of memory, which is discussed here: 

Possible memory leak - GeneratePendingTransforms/ImageMagick

Relevant code:

// From http://stackoverflow.com/a/12835966
// Pixel cache max size
$this-&gt;setResourceLimit(\Imagick::RESOURCETYPE_MEMORY, 256);
// Maximum amount of memory map to allocate for the pixel cache
$this-&gt;setResourceLimit(\Imagick::RESOURCETYPE_MAP, 256);


in craft/app/vendor/pixelandtonic/imagine/lib/Imagine/Imagick/Imagick.php at line 33, at top of smartResize() function.

Update:

If you are not on shared hosting but on a VPS (or otherwise have sudo access on your server, OR are able install ImageMagick + php-imagick from source), you can set memory caps for ImageMagick directly using a policy.xml file, e.g.:

&lt;policy domain="resource" name="memory" value="1GiB"/&gt;
&lt;policy domain="resource" name="map" value="1GiB"/&gt;
&lt;policy domain="resource" name="area" value="1GB"/&gt;
&lt;policy domain="resource" name="file" value="768"/&gt;