since the flesh creates paramters for all compiled in (rather than activated) thorns initially, this affects the default value that a thorn sees. Thorns seem to be initialized (and thus their parameter structures being created) in alphabetical order, which means that the thorn alphabetically '''last''' (How, I have no idea, the parameter handling logic seems a bit of a mess) will determine the default parameter value.
Attached is a parameter file to demonstrate this with Carpet and PUGH who both declare parameters periodic and periodic_[xyz] but differ in their defaults.
To demonstrate, create an executable with only Carpet compiled in and run the parameter file. Then look at the paramters in the checkpoint it creates eg.
h5dump -r -d /Parameters\ and\ Global\ Attributes/All\ Parameters output/checkpoint.chkpt.it_0.h5 | grep periodic
Do the same with an executable that contains both PUGH and Carpet. Notice that parameter values are now PUGH's defaults.
This can actually cause a runs to abort when recovering from a checkpoint when one switches from an executable with PUGH compiled in to one that does not. (Beyond the fact that some thorn might actually use it's parameters rather than the Carpet/PUGH pair where happily Carpet ignores these parameters and PUGH who actually used them gets to set the default).