You've probably heard about issues with the old mags in Gen 3 Glocks.
There was an issue with Glock 22 gen3 using a taclight if they did not have the 11 coil spring in the magazine. We had this issue with a few of our Glocks at our PD, Glock told us to check the mags, and we did have mags that did not have the 11 coil spring (I think ours were 10 coil, maybe it was 9, not sure). We switched to the 11 coil spring mags and the problems have dissappeared.
I can't begin to understand how an extra coil in the magazine can effect how the pistol functions in relation to a taclight, but it did work for us.