What happens when you use an inappropriate subnet mask for a Class C network?

Ok. I have never run into this before, so I have no idea what kind of issues this would raise. I just found a network which has the following setup
192.168.0.0/255.255.240.0.

It has been setup this way, I guess, for years. But apparently there have always been "issues." though I haven't gotten a clear picture of what they are. I personally have never seen this mistake before, so I am hoping someone can give  a quick summary of what kind of problems this would cause.
JesusFreak42Asked:
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Tim PhillipsWindows Systems AdministratorCommented:
Having a network setup this way won't "cause problems" in most circumstances.  It is actually handy because you now have a LOT of usable IP addresses (~4000 IPs).

Basically, here is your new IP network information:
First useable address: 192.168.0.1
Last useable address: 192.168.15.254
Broadcast address: 192.168.15.255
Subnet: 255.255.240.0  (also known as /20)

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JesusFreak42Author Commented:
OK. I couldn't remember if the Class A B and C settings were just "good practice" or if there were technical issues that they would cause.
Tim PhillipsWindows Systems AdministratorCommented:
Nope, they are just logical groupings for subnets, not rules.
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