Better backtracing to fix dodgy themes and plugins. Pop this into mu-plugins

gistfile1.txt

<?php
function sennza_backtrace() {
    echo implode( '<br>', explode( ',', wp_debug_backtrace_summary( null, 4 ) ) );
};
add_action( 'deprecated_function_run', 'sennza_backtrace' );
add_action( 'deprecated_argument_run', 'sennza_backtrace' );
add_action( 'doing_it_wrong_run',      'sennza_backtrace' );