Engineer turret upgrade skill doesn't let you build the second turret once you would have enough rank points and 4th skill unlocked. Respawning doesn't solve it nor does reselecting the skills.
Yeah, I almost feel like that bug is patch worthy by itself. So if there's a bug free fix out there, shoot.
When you consider the bugs in the initial Steam release, which lasted nearly 2 years, I don't really think this can be considered "super serious". It's not ideal but it's hardly game breaking.
Well not to harp too much more on this but, its game breaking in that it cuts the engineers balls off. He is half the man that he could be. one less turret, one less camera, and a lot less walls.