This is a good one. Introduced in v0.93. I can't reproduce this bug in neither Chrome nor Safari, but only on iPhone.
The Forms example at http://www.sencha.com/deploy/touch/examples/forms/ throws an error when rendering the NumberField in it and doesn't render anything that goes after it. I created a simple page and got the same error:
HTML Code:
<!DOCTYPE html>
<html>
<head>
    <meta charset="utf-8" />
    <title>NumberField</title>
    <link rel="stylesheet" type="text/css" href="/touch/resources/css/ext-touch.css" />
    <script type="text/javascript" src="/touch/ext-touch-debug.js"></script>
    <script type="text/javascript">
        Ext.setup({
            onReady: function() {
                var form = new Ext.form.FormPanel({
                    fullscreen: true,
                    items: [{
                        xtype: 'fieldset',
                        title: 'Number Field',
                        items: [{
                            xtype: 'numberfield'
                        }]
                    }]
                });
            }
        });
    </script>
</head>
<body>
</body>
</html>
The error is:
JavaScript Error on Line 13488 in ext-touch-debug.js
TypeError: Result of expression 'item' [null] is not an object.
Thanks.