Once in compatibility mode, pressing Enter to add comments no longer works. So there is something wonky going on with the scripts (happened on a recent change set that was rolled out?)
I can repro this at will and it causes all sorts of issues for me. Once I'm in compat mode, my tags look like crap Why do my favourite tags look screwy only on travel.se? and I can't use arrow keys to scroll after clickg a voting button https://meta.stackexchange.com/questions/110452/has-something-changed-recently-that-would-affect-focus-for-keystrokes . As you can see by those questions, it's been going on a while for me.
I"m using IE 8 on Win7, 64 bit. Without fail if I press Enter to submit a comment I am put into compat mode if am not already in it. Taking myself out of compat mode by clicking the broken page icon eliminates my screwy tags, keyboard focus problems, etc. I am close to being trained not to press Enter by this bug.
Update: Jan 4th/12 (or possibly a little earlier, I don't comment every day) something changed so that now submitting comments by clicking the button puts me in compat mode. This is so not an improvement. Any chance you can tell what's going on by looking at comment-related code that went live this year?
New Info - commenting on a question (with enter or the button) never puts me in compat mode. Commenting on an answer (again, with enter or the button) always does.
This is being caused by a "IE8 Specific" page layout error and the effects created can be stopped by doing the following.
To Stop compatibility view from recovering from page layout errors the Solution is simple.(although it took me awhile to figure out were the option was)
Open "Tools" menu then go to "Internet Options". Under the "Advanced setting" tab scroll down to The "Browsing" Section and Uncheck the option "Automatically Recover From page layout errors with Combatibility View"
This will not fix the layout incompatibilty with IE proprietary browsing visualization but there will be no unwanted loss of your individual Menu Items.
This bug, which just refuses to die, now has another symptom: no top bar showing my name, rep, badges etc on beta sites in compat mode. I got into compat mode by pressing Enter to comment. There was a comment on the answer already, and it was not upvoted.
On graduated sites the top bar doesn't go away, and I don't remember it ever going away with the sketchy theme on beta sites. I don't know what the fix for this is - whether to stop putting people in compat mode for no reason, or to behave better in compat mode - but I long for it.