Catchable fatal error seem to have fixed with cache clearing

[genestevo]

Hi Guys,
Had an issue a week or two ago with having too many expanded pages open in listcontent.php CallGuy provided the solution, thanks again. Now working collapsed by default.
Address is http://www.historypoints.org
Today however I had a call from an editor who said "All I did was upload a map and all the lights went out..."
This error showed when trying to open the pages section (and some others).
Catchable fatal error: Object of class cms_content_tree could not be converted to string in /home/mochdren/public_html/subdom_hp/lib/classes/class.contentoperations.inc.php on line 614

line 614 reads: $expr = 'content_id IN ('.implode(',',$explicit_ids).')';

lines 606 to 620 reads:
function LoadChildren($id, $loadprops = false, $all = false, $explicit_ids = array() )
{
$gCms = cmsms();
$db = $gCms->GetDb();

$contentrows = '';
if( is_array($explicit_ids) && count($explicit_ids) )
{
$expr = 'content_id IN ('.implode(',',$explicit_ids).')';
if( !$all ) $expr .= ' AND active = 1';

// note, this is mysql specific...
$query = 'SELECT * FROM '.cms_db_prefix().'content FORCE INDEX ('.cms_db_prefix().'index_content_by_idhier) WHERE '.$expr.' ORDER BY hierarchy';
$contentrows = $db->GetArray($query);
}

The big problem this time was that we lost the front end also. The site was effectively dead.
I had a root about and crossed my fingers as I hit the clear cache button. Hooray! It worked instantly and all is now normal again.

My question is this. How did clearing the cache solve what (felt like) seemed to be a calamitous event?
Seems like the quicker I get my finger out and swap to 1.11.9 the better.
Any advice greatly appreciated as always.
Cheers,
Eugene.