- Same OS, server: you likely control all the processes in it anyway, no untrusted code.

- Different OSes in different VMs, server: they likely run different versions of Linux kernel, libc, and shared libraries anyway. They don't share the page caches for code they load from disk.

- Browser with multiple tabs, consumer device: likely they might share common JS library code on the browser cache level. The untrusted code must not be native anyway, and won't load native shared libraries from disk.

- Running untrusted native code on a consumer device: well, all bets are off if you run it under your own account; loading another copy of msvcrt.dll code is inconsequential compared to the threat of it being a trojan or a virus. If you fire up a VM, see above.