at job disappears?

[Dieter]

FreeBSD 6.2
AMD64 (single CPU)
/var is FFS with soft-updates, on SATA.

/var/cron/tabs/root  contains:

	*     *       *       *       *           /usr/libexec/atrun

I had three at jobs queued.  They all call the same shell
script with different arguments.  First one runs fine.
Second one gets:

	atrun[3212]: cannot open input file: No such file or directory

And then the third one runs fine.

The machine is idle except for the at jobs.  No reboot, no fsck.
As far as I know, nothing should be mucking around in /var/at except
atrun.  Nothing to explain a file disappearing into thin air.