I performed a search on yahoo for an area school:
        walnut grove pleasanton
I clicked on the link for "Welcome to the Walnut Grove Web", which ended up with the URL:
http://www.pleasanton.k12.ca.us/walnut%5Fgrove/
This page has 2 applets on it, one of which failed to load on mustang (b34) and both of which failed to load on 5.0_01.

When I changed the URL to use an underscore instead:
http://www.pleasanton.k12.ca.us/walnut_grove/
the applets loaded and ran fine (on both 5.0_01 and mustang b34).

Note that a google search uses the correct (underscored) link, so to duplicate this use Yahoo instead.

it seems that ascii substitution is common in emails and web, so plugin ought to handle URLs which contain such characters.

Here is the out put in the java console after the applet failed on mustang (note that some of this output is there even on the working version, but the stack traces are only there when it fails):

Counter applet - Cameron Gregory - http://counter.bloke.com/
***Warning: Counter will not Reset:
***User specified URL: http://www.pleasanton.k12.ca.us/walnut_grove/index.html
***does not match real: http://www.pleasanton.k12.ca.us/walnut_grove/main.htm
java.net.UnknownHostException: www1.counter.bloke.com
at java.net.PlainSocketImpl.connect(Unknown Source)
at java.net.SocksSocketImpl.connect(Unknown Source)
at java.net.Socket.connect(Unknown Source)
at java.net.Socket.connect(Unknown Source)
at java.net.Socket.<init>(Unknown Source)
at java.net.Socket.<init>(Unknown Source)
at Counter.c(Unknown Source)
at Counter.<init>(Unknown Source)
at Counter.cm(Unknown Source)
at Counter.start(Unknown Source)
at sun.applet.AppletPanel.run(Unknown Source)
at java.lang.Thread.run(Unknown Source)
Magic Buttons v.3.2
© Image Intelligence Ltd.
http://www.imint.com
Licenced to:
Michelle Coldani
http://www.pleasanton.k12.ca.us/walnut_grove
Actual Location
www.pleasanton.k12.ca.us/walnut_grove/
unavailable
Counter applet - Cameron Gregory - http://counter.bloke.com/
***Warning: Counter will not Reset:
***User specified URL: http://www.pleasanton.k12.ca.us/walnut_grove/index.html
***does not match real: http://www.pleasanton.k12.ca.us/walnut%5Fgrove/main.htm
java.net.UnknownHostException: www1.counter.bloke.com
at java.net.PlainSocketImpl.connect(Unknown Source)
at java.net.SocksSocketImpl.connect(Unknown Source)
at java.net.Socket.connect(Unknown Source)
at java.net.Socket.connect(Unknown Source)
at java.net.Socket.<init>(Unknown Source)
at java.net.Socket.<init>(Unknown Source)
at Counter.c(Unknown Source)
at Counter.<init>(Unknown Source)
at Counter.cm(Unknown Source)
at Counter.start(Unknown Source)
at sun.applet.AppletPanel.run(Unknown Source)
at java.lang.Thread.run(Unknown Source)
Magic Buttons v.3.2
© Image Intelligence Ltd.
http://www.imint.com
Licenced to:
Michelle Coldani
http://www.pleasanton.k12.ca.us/walnut_grove
Actual Location
www.pleasanton.k12.ca.us/walnut%5fgrove/
unavailable
Magic Buttons v.3.2
Counter applet - Cameron Gregory - http://counter.bloke.com/
© Image Intelligence Ltd.
***Warning: Counter will not Reset:
***User specified URL: http://www.pleasanton.k12.ca.us/walnut_grove/index.html
***does not match real: http://www.pleasanton.k12.ca.us/walnut%5Fgrove/main.htm
http://www.imint.com
Licenced to:
Michelle Coldani
http://www.pleasanton.k12.ca.us/walnut_grove
Actual Location
www.pleasanton.k12.ca.us/walnut%5fgrove/
unavailable
java.net.UnknownHostException: www1.counter.bloke.com
at java.net.PlainSocketImpl.connect(Unknown Source)
at java.net.SocksSocketImpl.connect(Unknown Source)
at java.net.Socket.connect(Unknown Source)
at java.net.Socket.connect(Unknown Source)
at java.net.Socket.<init>(Unknown Source)
at java.net.Socket.<init>(Unknown Source)
at Counter.c(Unknown Source)
at Counter.<init>(Unknown Source)
at Counter.cm(Unknown Source)
at Counter.start(Unknown Source)
at sun.applet.AppletPanel.run(Unknown Source)
at java.lang.Thread.run(Unknown Source)

###@###.### 2005-05-17 22:22:58 GMT