In our office we use 172.x.x.x/8 as the internal network however I've noticed that only 172.16.0.0 – 172.31.255.255 is registered for internal use. Do you think that this will cause any problems in the future?
It might cause two types of problems:
If possible shift to 10.0.0.0/8. Only first octet will change rest bytes can remain as it is.
You won't be able to access public ip address that are in your /8 subnet (The rest of these are not reserved according to RFC3330) because they will never make it your gateway. The reason is that since the host will think they are on the network, they won't send it to the gateway. If you have a router interface with that mask, then the router will always send requests it receives to that interface because directly connected routes have the lowest administrative distance.
You really do need to fix this, the easiest fix would be just to change the subnet mask on DHCP and any servers to /12 or 255.240.0.0.
Kept for your information, but I read the question wrong the first time:
If you wish to learn more, subnetting is the topic you need to read about. Evan wrote a nice introduction to the topic in his answer here.