really strange one, this.
I have a simple channel with one matrix field, which contains a single fieldtype of 'date'. It works in the control panel OK, but if I try to run the tag pair on the front end I get:
Fatal error: Class 'EE_Fieldtype' not found in /path/to/_system/expressionengine/third_party/matrix/ft.matrix.php on line 14
This is EE v2.6.1 - Build Date: 20130506, and Matrix 2.5.5 or 2.5.6. But I have both Matrix and EE working OK on other projects at these versions so I'm stumped.
Anyone have any suggestions better than my "start again" one?