Currently it's possible to submit some values via Long Textfield<Long> that are greater than Long.MAX_VALUE. This will produce converted input and model update with value of Long.MAX_VALUE
I'm not sure what the behavior should be - imho throwing ConversionException seems fair as the input isn't a valid Long.
The reason seems to be precision loss during Double.valueOf(input) execution while converting, and then comparing to Long.MAX_VALUE using Long.doubleValue() in AbstractNumberConverter, which by casting leads to to the same precision loss and the numbers are seemingly equal during comparison of ranges.
Maybe using BigDecimals for parsing could help here.
The quickstart is available at https://github.com/zeratul021/wicket-number-conversion.
For the fastest demonstration I extended Wicket's longConversion() test-case in ConvertersTest: https://github.com/zeratul021/wicket-number-conversion/blob/master/src/test/java/com/github/zeratul021/wicketnumberconversion/ConvertersTest.java#L300