Things Players Aren't Actually Much Good At

[Concerned Ninja Citizen]

Out of curiosity, what level was the party, how long was the room, and could they get line of sight to the lever without entering the room?

[Demelain]

Party is level 4, The room was 30x40 ft and they had clear LoS to the lever.

[Keldar]

I said lever, that was a typo.

EDIT: No, not a typo. My FireFox is auto-correcting me? WTF FireFox.

EDIT 2: Disabled auto-correct. Should be fine now.

Well that explains why it happened more than once.  And why I had to ask.

[veekie]

Then they probably just forgot all about the golem.

[MirddinEmris]

Most of them really don't know when they should back off

[Keldar]

Then they probably just forgot all about the golem.

Yea, its easy to forget things like that.  Especially if its the kind of thing no one is liable to write down on a character sheet.  Which is why I try to use note cards and minis for things like that.  A physical object for the players is a lot harder to forget than words they only heard once or twice.
Of course as a DM, that is quite a bit of extra effort so it can be easy to slack off.   

[RobbyPants]

I would reiterate 1, 2, and 5.

Other's I've seen from my group include:

7. Not thinking of position tactically, particularly regarding things like flanking, breaking charge lanes, and avoiding or setting up for AoOs.

8. Not thinking of what types of defenses that an opponent might have that would likely be weak (Will for fighty types, Fort for some casters, touch AC for big things, grapple for small things) and picking spells or attacks to target those. Granted, a lot of this is learned behavior and is easier if you've seen a Monster Manual, but it is something you should pick up from the PHB rules after rolling up two or three PCs.