Misfire / Check Engine light

So, this afternoon I attempted to change the spark plugs out in my 2001 Sport. It's a 6-cyl. auto, and has the distributorless ignition.

I was able to remove the coil 'rail' from the plugs, but not in any way meaningful enough to actually get at the two plugs nearest the firewall; I couldn't see how to get the rail off of its connector without a mirror, which I didn't have with me. Because of this, I ended up not changing the plugs (though I did tighten down #1 a tad, which was slightly loose) until I had more time to figure out how to extract the rail.

The rail was replaced, and appeared to tighten down on all four bolts without any trouble. However, after about 20 miles' driving, a slight stumble developed at idle - power delivery is the same as before, just that there's a noticeable 'bump' at idle every few seconds. This was followed by the check engine light coming on. No problems when rolling - I only feel anything being off at idle.

My guess is that I disturbed something in my fiddling, but can't see anything obvious (I checked). Does anyone know what I might have screwed with? I've done my own basic maintenance for years (plugs, wires, coolant, oil), but have never had anything quite like this happen as a result. Then again, it's my first distributorless-ignition vehicle so I guess something was bound to go wrong. :doh:

this is an obd2 vehicle, so if you have access to a scan tool you can tell exactly which clylinder(s) misfire. it could be as simple as one of the spark plug boots isnt seated on the plug correctly or a cracked or misgapped plug. sometimes it can be difficult to reinstall the coil back on the plugs especially with the limited room around cylinders 1 &6. also on some of the harder to install plugs i have seen the spark plug gaps get knocked closed upon installation.