Help with a bug in nroff sought - (nf)

[jeff at ism780.UUCP]

#R:sri-arpa:-65000:ism780:14400004:000:489
ism780!jeff    May 15 13:44:00 1984

>  From: Bernie Cosell <cosell at bbn-labs-b>
>  Subject: bug explanation sought
>  To: nroff at bbn-labs-b
>  In trying to debug a bunch of macros, I was going nuts because I apparently
>  blundered into a bug in nroff.  Can anyone confirm just exactly what this bug
>  is and how it works?
>      .de X
>      .so /etc/motd
>      .tm it never gets here
>      ..
>      .X

Is the input file >64k?  Are you running on a 16-bit machine?
The n/troff sources neglect to declare lseek() as long.