Normally you should have no problems nesting DURING/HANDLER blocks. There is a built-in limit to the depth of a stack of nested handlers so failing to end a DURING block will eventually cause an access violation. DURING is also one of the places where a crash is likely if non-thread-safe versions are used in a threaded environment.
In a .NET environment, DURING is implemented using setjmp/longjmp. This makes it completely illegal in .NET managed code. The Adobe PDF Library offers a Java and .NET interface serves as a .NET wrapper for the Adobe PDF Library.
Some customers have built their own .NET wrappers for use with the Library. We recommend that .NET users who are building their own wrappers for APDFL should create their application using non-managed code, and use only a thin wrapper of managed code to interface to higher level applications.