I think this deserves its own thread.
Met the "Mini Sentry" before?
It's a nice, little, yet effective exploit. I'll tell you later about it.

What?! A level 3 sentry while you have the gunslinger equiped?!Well, yea. That's actually a part of the exploit!
That sentry is blinking! It has something to do with the combat sentry!Not really.
Dawww! It's so adroable!I know!
So, how do I create a mini sentry?It's simple, really.
First: Equip Wrench
Second: Select Toolbox & Sentry, don't place it yet!
Third: Equip Gunslinger
Fourth: Place sentry
Fifth: Upgrade
Sixth: Redeploy
Pro and contra:Pro:
A normal level three sentry needs 10 seconds to build itself up. A mini sentry needs half of the time! Just five seconds until it's ready to fight for you!
Another pro would be that it stays the same. It's a normal level three sentry, it still has the same HP and damage power!
Contra:
None! It's perfect! You just have to love it!
You can't heal it though.