I believe this is the same bug as bug report 11803, but I think the scope is wider than initially reported.

The bug: set_error_handler does not trap an error if I try to call a method that does not exist.

This is causing grief when a PEAR-style method returns one type of object on success and another on failure.   The code expecting the success will call a non-existant method.

Here's some code to emulate the problem:
<?php

// set up my custom error handler
set_error_handler("handleError");

function handleError($errno, $errstr, $errfile, $errline, $obj)
{
  print "<b>My Error Handler:</b> $errstr on line $errline";
}

// define a really basic class
class Foo
{
  var $i;
}

$myFoo = new Foo();

// call a non-existant property (this calls the errorHandler OK)
print $myFoo->choo;

// now call a non-existant method (this bypasses the errorHandler!!!)
$myFoo->bar();

?>