I've encountered a very strange error in ASP.NET.  I've been able to reproduce it successfully and consistently in a test project.

Normally if you hit the back button, the page you go back to does not postback.  However, if on the second page you click a link that opens a new browser window (a hyperlink with target="_blank") and then click the back button on the original browser window, the first page will postback even though you only pressed the back button. 

SmartNav is not enabled.  This is on v1.1 of the framework.

Anyone ever seen/heard of this?  I can't find anything online, or in the newsgroups, or on the Microsoft support site.

UPDATE:  This is not an ASP.NET error, it is an Internet Explorer error.  Unfortunately I cannot have people switch browsers since it is a public site, but at least I know what's going on!  Thanks to those who helped.