Mostly an observation here, but I was shocked to see this happen.
In ESXi 6.5, when upgrading VMware Tools from the web GUI, it automatically reboots your system without any warning that it's going to take this action.
After noticing this and reading around, I see that it's fairly well documented that in order to perform an installation without reboot, you need to enter in the appropriate command switches. That said, when I initially installed VMware tools (which I've done on various ESXi versions from 5.0 through 6.5), it always essentially "loads a dvd" into the guest, which requires me to run the executable manually. After running the executable, it notifies me that the application requires a reboot of the system, in order to complete the installation, and allows me to reboot right then, or to postpone that until I'm ready.
I fully anticipated this to be the same case when performing a VMware Tools upgrade, and instead was met with a middle of the day production server outage due to the unexpected and unannounced reboot.
While I can appreciate that there is documentation to support a method of upgrading the tools without a reboot, I am failing to see any documentation to support that a reboot would be required in all cases, or that it would do this without warning.
Is performing a reboot without warning an intended action? It seems like it would be fairly easy to make one more little pop-up before actually upgrading the tools to let you know that it's about to do that. Nearly every piece of software that I've ever installed (and all of the good ones, up until now) have all given me some sort of warning that a reboot is imminent, allowing me the opportunity to decline in some way. Even if declining the reboot makes it such that nothing happens at that point, at least then it avoids the disaster of unintentionally rebooting a server without anything prompting you indicating that it's going to do that.
This seems like an oversight in the web GUI for ESXi 6.5, and I highly recommend that it be updated to reflect the necessity of the reboots that will be forced, giving you the opportunity to opt out of them before disaster strikes.