I'm running KDE 5.10.4 on Fedora 26. Whenever I'm in Konsole and press the Win/Super key along with any key that's not bound to a system-wide KDE shortcut, such as 'I', it sends some weird input that: (1) prints some contents from /etc/hosts (but not the whole contents of /etc/hosts), and (2) writes "si" (or eg. "sd" if I pressed Super-D instead) on the next line. For example:

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[eru@keionbu ~]$
::1                      localhost4.localdomain4  localhost.localdomain
localhost                localhost6               
localhost4               localhost6.localdomain6 
[eru@keionbu ~]$ si

The first line here being where I press Win+I. It should be noted that this also happens if there's already text on the first line. It also doesn't seem to happen with other terminal emulators like xterm or terminator.

I tried changing Konsole's keybindings and the Keyboard setting (between Xfree4, Linux, and Solaris), but it did not seem to solve the issue. I also tried changing the various settings under System Settings > Input Devices > Keyboard > Advanced that claim to pertain to Alt/Win key behavior, but to no avail.

Any ideas how to fix it besides just finding another terminal emulator? It's pretty annoying because I occasionally accidentally press Super instead of Alt, and it causes this weird input/output.

That is totally weird. I can confirm it in KDE 4 - hitting various unbound Win+key combinations prints the currently-configured list of hosts; so the behavior has been there for a while.
I'd recommend filing a bug to bugs.kde.org.

More details:
I tried using raw sh and hitting e.g. Win+i emitted the characters: "^X@si" (without printing the list of hosts). So I tried the "^X@" combo as ctrl+x followed by @ in bash in different terminal emulators and got the same behavior: printed the list of hosts; so that must be a feature provided by bash. To summarize: Konsole is interpreting the Win+key combination to emit the ctrl+x+@+s+key sequence, bash is interpreting the ctrl+x+@ part of that sequence to print the list of hosts and then treating the rest of the sequence as the beginning of the next command.

You could certainly try other shells instead of bash. A quick google search for bash alternatives will pop up several.