Commas are allowed in the filename part of a URL, but are reserved characters in the domain*, as far as I know.
What version of IE are you using? I’ve come across the odd report of IE5.5 truncating URLs on a comma (link here, but have tested URLs with commas in IE7 and it seems to be OK, so if there was an IE bug, it doesn’t seem to be there any more – could it be an IIS issue?
I’m wondering if the page error is due to a rule failure with the mod_rewrite – can you post the rule which is matching multiple ids and passing them off to your Foo.aspx? Is there any chance that it’s only matching Foo.N,N, and failing on more commas?
* From the URI RFC:
2.2. Reserved Characters
Many URI include components consisting of or delimited by, certain
special characters. These characters are called “reserved”, since
their usage within the URI component is limited to their reserved
purpose. If the data for a URI component would conflict with the
reserved purpose, then the conflicting data must be escaped before
forming the URI.reserved = ";" | "https://stackoverflow.com/" | "?" | ":" | "@" | "&" | "=" | "+" | "$" | ","The “reserved” syntax class above refers to those characters that are
allowed within a URI, but which may not be allowed within a
particular component of the generic URI syntax