cutting too deep

purchased a pattern and plugged it into the carver, started carving but a little over 3/4 through carve, bit carved all the way through board. I've carved nine other established signs, same thickness without any issues. I thought it might be pattern so I tried another project and the carve began normal but half way through, the bit went all the way through the wood again. Anyone have this issue and if so what is wrong?

I would start by re formatting the card. Other things to look at, ffc cable torn/cracked or loose in the connector on the back of the Z truck, dirty encoder disc in the Z motor,