File size weirdness on a 486
My problem was reported by an alert Chemistry lab technician looking after a handful of student PCs (386 PCs from one manufacturer and 486s from another) all running MS-DOS 5. He'd experienced usual glitches and subsequently observed that the reported file size for some COM files was different depending on the PC; some of the 486s reported differently from ostensibly identical models and from all of the 386s. Naturally he suspected a virus but all of the PCs were running the same AV product, VIS Utilities which had been updated at the same time. And he'd nailed it down to some of the 486s.
I struggled to diagnose the problem before observing that some COM files changed size when copied to a floppy disk and examined on "good" and "bad" PCs. Then I turned off the AV software, VIS Utilities. And the problem went away. The "virus problem" was anti-virus software.
My guess at the time was that the identical 486s had a motherboard revision or cache/RAM from different manufacturers.