SET WEIRD ON

I have an error appearing - in development mode - without rhyme or reason and I can't trap it in the debugger other than to prove where its happening. The code snippet is simple enough:

CASE ALIAS()='ORDHEADS'
INDEX ON ACCREF TAG ACCREF [error occurs here - "Variable Date is not found']
INDEX ON PADL(ORD_NUM,8,'0')+PADL(SUB_NUM,4,'0') TAG ORD_NUM ADDITIVE
etc


The error reported is 'Variable (whatever) not found'. (the first missing variable is 'date' - nothing to do with 'ORDHEADS' or ACCREF.)

The ordinary dev-time error handler does not appear (with its 'Cancel Suspend etc' options); though it is 'available' for all other errors.
(and even if I insert ON ERROR immediately before the offending line, the same messagebox comes up)

I just get a box with 'Variable (whatever) not found' with only 'OK' and 'Help' options. (The sort of window we get running a compiled exe and your built in errorhandler fails to trap the error.)

It then reports a series of variables not found with considerable repetition (the closest behaviour I can compare it too is the sort of thing we see if a form refresh isn't finding a bunch of variables. There is no 'refresh' code running in this case). I know exactly where these variables belong (in a completely different form and table)

When its finished whining about a dozen times, it then carries on as though nothing has happened.

All the while, neither the alias nor the form have changed and they are the correct ones. The variables being reported as not found aren't required at this point.

So far I've tried various including:
setting ON ERROR just before the DO CASE in case some alien errorhandler had been switched on somehow.
I've switched the lines so that the second index is created first. Produces exactly the same error.
setting a breakpoint on ERROR() to see if I can force it to stop at the error itself. No value appears in ERROR()

Obviously I'm doing something silly elsewhere in my code, but with bizarre behaviour like this I can't figure out how to debug it. I can't expect anyone else to guess what the coding problem is, but can anyone give me a clue on how to begin to diagnose it!

Thanks

Harry