Steps to reproduce:

1.- Create a currency with this format: ###.###,00
2.- Create a product and assign this currency with the previous format: 2,65 for example.
3.- Render the product page. This will not show the price because of the formatting.
4.- Try to add this product to the cart.

You get a blank page and this exception:

info.magnolia.rendering.engine.RenderException: java.lang.RuntimeException: Can't read value of nodedata shopProducts:/edwinShop/calcetin1/prices/0/price[String]
.....
Caused by: java.lang.RuntimeException: Can't read value of nodedata shopProducts:/edwinShop/calcetin1/prices/0/price[String]
at info.magnolia.cms.core.DefaultNodeData.getDouble(DefaultNodeData.java:146)
.....
Caused by: javax.jcr.ValueFormatException: not a double: 2,65
at org.apache.jackrabbit.spi.commons.value.AbstractQValue.getDouble(AbstractQValue.java:287)
.....
Caused by: java.lang.NumberFormatException: For input string: "2,65"
at sun.misc.FloatingDecimal.readJavaFormatString(FloatingDecimal.java:1250)
at java.lang.Double.parseDouble(Double.java:540)
at org.apache.jackrabbit.spi.commons.value.AbstractQValue.getDouble(AbstractQValue.java:285)