<c:set var="myVariable" value="<%= myObject.get("key")%>"/>
Yields the following on several app servers (including Tomcat 6.0.28):
01-Apr-2011 12:24:57 org.apache.catalina.core.StandardWrapperValve invoke SEVERE: Servlet.service() for servlet action threw exception org.apache.jasper.JasperException: /my.jsp(25,44) Attribute value myObject.get("key") is quoted with " which must be escaped when used within the value
It turns out the JSP 2.0 specification clearly states that this is illegal and thus all application servers that claim to be JSP 2.0 compliant will fail to compile such JSPs.
The funny thing with Tomcat, is that they changed the behaviour of their JSP compiler in a maintenance release, namely 6.0.17 (with a regression bug fixed in 6.0.26) as they then already claimed to be JSP 2 compliant, but was not. Hence, this was a bug according to their 6.0.0 feature list and was thus fixed in 6.0.17 (and again in 6.0.26, released 2010-03-11).
Thus, if you're using Tomcat and haven't upgraded it since march last year, you stand the chance that your JSPs that run flawlessly on your app server, not will do so on others'.