This was reported on the mailing list.
In certain places, hitting the refresh button rapidly can cause a null pointer exception.
It is believed this is related to a JDK 1.5 bug:
Tapestry makes a few uses of ThreadLocal that are consistent with this pattern. ThreadLocals are used to connect service proxies to perthread scope services.
We will locate all useages of ThreadLocal and, alas, synchronize access to them.
More discussion: http://markmail.org/message/7bwztu66paz2cfqm