Nice anology Cor.
I generally roll my dice in the open - only every once in a while making secret Detection rolls when I don't want to tip players off. If anything, if a fudge occurs it's because I (usually subconsiously) pull an NPCs punch in some way. More often than not, I'm just forgetful or a bad tactician and the players get an unexpected benefit from my untimely goof. It's also why I don't mind an occassional fudge or impromptu reskin of abilities* - I'm not a perfect person and a tweak here or there to compensate for that doesn't seem out of line.
Also, in non-D&D games, I do a lot of enemy stats off the cuff - for two reasons. The foremost is that generally, other games don't have the breadth of premade NPCs/monsters that D&D does. Second, most other games don't have a time-consuming creation requirements that D&D does. I can generally decide on such NPCs on the fly and do a decent job.
If the players go off in left field in a game (as which has happened in my current Hunter game) I have no compunction about making an NPC enemy on the spot and rolling with it, rather than grind the game to a halt so I can waste play time on fully statting out an NPC that's going to get rolled in 5-15 minutes of combat. (There's no way in hell I'd do that in the middle of a 3.5/Pathfinder game.)
* In our Rise of the Runelords game, the party was supposed to face a BBEG goblin on the back of a lizard. Lacking an appropriate miniature, I used the goblin spider Birthright mini I had in its place, describing the goblin as mutating into this form for battle. It ran mostly the same, but the reaction it drew by this impromptu tweak felt like it made the battle more exciting - as well as inspiring some interesting BS explanations of what caused the mutation after the fight.