This may prove to be a stupid question, but I've scoured in internet and found nothing to that gives me a concrete answer.
I'm curious as to potential consequences of a file server being rebooted or crashing when users have files open. Now common sense would tell me that as long as they aren't being saved or written to as the reboot happens, corruption is unlikely. I have been forced to reboot my server a couple times when users had files open (in both read and read/write mode) but never heard of any complaints from users saying they had lost files or they had issues. I mean if a file server goes down and a file is open, that (theoretically) means that they won't be able to save to the network share until the machine comes back online, right?
My question is then, is it safe to reboot a server if files are open by end users?
Your common sense serves you well; although I always try to warn people prior to rebooting a server so they can save and close as long as there is no active I/O happening nothing will happen. This is of course also assuming you're shutting down and rebooting, and not just yanking the power cord (in which case you have cache flushing issues to worry about).
Rebooting a server using the system tools will never corrupt the file system, but can corrupt the integrity of the date inside the files. E.g. if you have an application that uses 2 files and you have references in one file to data from the other file, and you reboot your server, you can break the reference if you reboot your server. Also, you can have a corrupted file if you are rebooting the server and a transaction to a file (an operation that takes the data from the file from a stable state to another stable state) did not complete.
Swap files wont be closed (not really bad, just annoying), if the files are actively writing they could be corrupted.