Unique Error Nightmare

I am working on a web application that uses SQL Anywhere 10. I am in the process of porting the DAL to NHibernate. I wrote a custom driver as the existing one only supports v9 -I'll contribute it once I'm sure that it's working OK.

Everything was going great until Fri afternoon. Some of the code that I haven't ported yet invokes ExecuteScalar() on the IDbCommand.

Suddenly the web app stopped working and I saw this in the Event Log:

Event Type: Error
Event Source: .NET Runtime
Event Category: None
Event ID: 1023
Date: 10/15/2007
Time: 10:59:48 AM
User: N/A
Computer: xxx
Description: .NET Runtime version 2.0.50727.1378 - Fatal Execution Engine Error (7A09239C) (80131506)

I resorted to the debugger (this is hard for a TDD advocate!) and when I used the Immediate Window to inspect the results of the call (Intellisense was not invoking) I saw:

"Cannot obtain value of local or argument "xxx" as it is not available at this instruction pointer, possibly because it has been optimized away."

I wrote a Console app and I see yet another error:

FatalExecutionEngineError was detected
Message: The runtime has encountered a fatal error. The address of the error was at 0x79f63d6d, on thread 0x1554. The error code is 0xc0000005. This error may be a bug in the CLR or in the unsafe or non-verifiable portions of user code. Common sources of this bug include user marshaling errors for COM-interop or PInvoke, which may corrupt the stack.

The wonderful thing is that the problem is not reproducible on any other machine other than my laptop. Don't you just love being a developer!