What are the secret achievements in Day of the Tentacle

The update notes say:

• Added 4 more achievements along the same lines as the "Medium Rare" achievement added last week
• Renamed "Medium Rare" achievement to "Medium Well"

Since they are along the same lines as the previous achievement, they mean the player should complete both the light world and the dark world of a chapter, and the sum of the best times for that chapter should be lower than some threshold. Here are the descriptions of each one:

• Rare - Spend as little time as possible in The Forest (around 320 seconds)
• Medium Rare - Spend as little time as possible in The Hospital (around 430 or 460 or 490 seconds)
• Medium - Spend as little time as possible in The Salt Factory (around 570 seconds)
• Medium Well - Spend as little time as possible in Hell (around 500 or 550 seconds)
• Well Done - Spend as little time as possible in The Rapture (around 740 seconds or more)

About the old secret achievements:

• Suffragette - Complete the light world of the Cotton Alley
• Seneca Falls - Complete the dark world of the Cotton Alley
• Well look at you! - Input the petaphile code to play with Tofu Boy

Sources:

• Super Meat Boy Wikia
• SteamStats
• Steam Forums

Note about the times for the achievements: Many people use Super Meat Boy Stats (old repo at Google Code, new repo at GitHub) program to find out how much time they have spent on each chapter. However, the reported total time is not exactly accurate because "it counts 15 secs per completed warp zone", and people posting their total times didn't mention if the warp zone time has been subtracted or not. For this reason, the times I posted above might be quite inaccurate.

From the support article regarding achievements. I've bolded the items that could apply:

Achievements are not recorded while playing in Commentary Mode, Offline Mode or when sv_cheats is set to 1. If you would like to ensure that your achievements are tracked correctly, please do not play your game in any of these modes.

If you notice that stats or achievements do not appear to be tracked, or previously attained stats and achievements are missing, that is a good sign that either stat tracking is temporarily down or that you may have an Internet connection issue. During this time, your stats and achievements may not be recorded. Unfortunately, that means they will not appear later when everything returns back to normal.

This is what I would assume first would be the culprit. Is it possible that you could've lost connection to Steam at the time you acquired the Terror Gauntlet (I don't have the game so I don't know what triggers this)? If so, it may have missed that "trigger" and it would not have registered with Steam. See this question, though note that there are exceptions to the rule. While the majority of games do not sync achievements that have been earned in offline mode, there are certain games which do, such as Borderlands. However, you'd need to look that up on a case-by-case basis.

Occasionally there may be issues with attaining certain achievements - in these cases, you will need to wait for a patch addressing this problem in order to attain them.

Very rarely certain achievements will be reset or adjusted, typically following a patch that is intended to fix issues with that achievement.

In any cases where you are missing achievements, they unfortunately cannot be restored. You will need to re-attain achievements that have been lost.

After doing some research, it seems that in regards to Darksiders specifically, there seem to be several threads about the same issue.

Some suggestions from people include:

• Don't alt-tab after you start playing, because there's some sort of crash that glitches achievements. Some people also said do not use the Steam overlay (shift-tab by default) because that crashed the earlier version of the game's connection to Steam as well, though from reading, that's supposedly been patched.
• Save before an achievement trigger/event so you can reload and try again if it fails to take the first time
• Shut down Steam, restart it and immediately play Darksiders before any other game.

Note that I don't own the game, so I can't confirm any of these methods myself. In any case, if you have missed the "gimme" achievements, there's unfortunately no way to re-trigger them short of loading up a game save before the trigger and playing until it unlocks or playing through the game again.