In my case I took the upper manifolds off to instal the Needswings catch can and didn't use any locking sealant on the bolts when I put them back on. During the next track day the car was running fine at WOT but a little strange at idle. On the way home I got a code. Poking around I discovered the bolts had backed almost all the way out. Another time I broke one of the bolts. Anyway, I've had them off 4 times in all and if you don't do everything right something will leak. If you've never had them off don't worry. The symptoms are an idle that just isn't perfect. A bog or hesitation on gentle throttle openings. A "cough" if you lift from WOT suddenly at high rpm. At least that's what I've observed. One of my leaks was from forgetting to instal the cork O rings under the outside bolts. One of those oversights that can occur. All I can say is if you have them off for any reason, use something like locktite blue and follow the instructions. (I didn't once and guess what happened even with the blue)

Les