Computer was acting odd when I got home

I was out for a few hours this afternoon. When I returned the CapsLock LED was on but the shift state was off; pressing CapsLock turned its LED off but CapsLock on. Alt and Ctrl didn't work: I couldn't change virtual terminals. Attempts to su failed in either Caps state (the password was rejected?). I pressed the power button, which Linux handled, turning off gracefully (elogind-daemon records this in /var/log/messages). A capture of an audio stream that crond had started 5 minutes before I pressed the power button had worked. What happened? There's nothing amiss in the logs.

Toshiba Satellite C855, about 10 years old.

Try another keyboard or if you have a live distro see if keys work there?

Not helpful at all, but a very similar thing just happened to me: all by itself Caps Lock turned on and wouldn't turn off, Shift stuck and wouldn't release (and so it is looked like caps lock is reversed and ctrl-alt-Fn wouldn't work), autorepeat stuck. Disconnecting and re-connecting the keyboard didn't change a thing, couldn't do much with it and rebooted - and it fixed everything. Found this in the logs:

Feb 23 12:15:49 server kernel: [1808407.237782] atkbd serio0: Spurious NAK on isa0060/serio0. Some program might be trying to access hardware directly.
Feb 23 12:15:49 server kernel: [1808407.246821] atkbd serio0: Unknown key released (translated set 2, code 0x62 on isa0060/serio0).
Feb 23 12:15:49 server kernel: [1808407.246829] atkbd serio0: Use 'setkeycodes 62 <keycode>' to make it known.
Feb 23 12:15:49 server kernel: [1808407.255829] atkbd serio0: Spurious ACK on isa0060/serio0. Some program might be trying to access hardware directly.
Feb 23 12:15:49 server kernel: [1808407.291870] atkbd serio0: Unknown key released (translated set 2, code 0x6e on isa0060/serio0).
Feb 23 12:15:49 server kernel: [1808407.291876] atkbd serio0: Use 'setkeycodes 6e <keycode>' to make it known.
Feb 23 12:16:01 server kernel: [1808419.111319] atkbd serio0: Unknown key pressed (translated set 2, code 0x71 on isa0060/serio0).
Feb 23 12:16:01 server kernel: [1808419.111327] atkbd serio0: Use 'setkeycodes 71 <keycode>' to make it known.
Feb 23 12:16:01 server kernel: [1808419.301609] atkbd serio0: Unknown key released (translated set 2, code 0x71 on isa0060/serio0).
Feb 23 12:16:01 server kernel: [1808419.301617] atkbd serio0: Use 'setkeycodes 71 <keycode>' to make it known.
Feb 23 12:16:02 server kernel: [1808419.928953] atkbd serio0: Unknown key pressed (translated set 2, code 0x64 on isa0060/serio0).
Feb 23 12:16:02 server kernel: [1808419.928960] atkbd serio0: Use 'setkeycodes 64 <keycode>' to make it known.
Feb 23 12:16:10 server kernel: [1808428.770885] atkbd serio0: Unknown key pressed (translated set 2, code 0x64 on isa0060/serio0).
Feb 23 12:16:10 server kernel: [1808428.770893] atkbd serio0: Use 'setkeycodes 64 <keycode>' to make it known.
Feb 23 12:16:14 server kernel: [1808432.748002] atkbd serio0: Unknown key pressed (translated set 2, code 0x71 on isa0060/serio0).
Feb 23 12:16:14 server kernel: [1808432.748004] atkbd serio0: Use 'setkeycodes 71 <keycode>' to make it known.
Feb 23 12:16:15 server kernel: [1808433.150518] atkbd serio0: Unknown key released (translated set 2, code 0x71 on isa0060/serio0).
Feb 23 12:16:15 server kernel: [1808433.150525] atkbd serio0: Use 'setkeycodes 71 <keycode>' to make it known.

Keyboard is a old and cherished bit of pre-windows hardware: AT (sic!) connected through a KVM switch to a PS/2 port - many thing that could go wrong here (although it never happened before), so at first I didn't bother and then read this eerie similar message. ROTM?