I found this error while trying to log onto the TeraGrid ("Tara Grid") with gsissh (google "teragrid single sign on" for context).
percent_expand: NULL replacement
Apparently this is a well-known bug with Mac 10.5's ssh. The workaround is to append "-i ~/.ssh/id_rsa" or "-i ~/.ssh/id_dsa" to the command line, like so:
gsissh login-hg.ncsa.teragrid.org -i ~/.ssh/id_rsa
I got this from http://www.nabble.com/ssh%3A-percent_expand%3A-NULL-replacement-to13439207.html