My guild has recently started working on Heroic Atramedes. One of the key things to the fight is dealing with Searing Flame, which is a channel by the boss that does hideous amounts of damage to the raid and has to be interrupted by clicking on a gong. You cannot click early or it just delays the Searing Flame and wastes one of your very limited gongs. The trouble is that Searing Flame ticks extremely quickly and even a half second delay pretty near guarantees deaths of raid members either to the Searing Flame itself or a follow up AOE attack. When you must click in a time window of significantly less than 1 second or the raid wipes you have a great recipe for an extremely random encounter. Due to totally normal latency and realistic human reaction time you are simply going to have regular failures no matter what you do. Even on normal mode we found the mechanic of mandatory interrupts on a cast that is exceedingly quick (and without a proper UI notification, no less) really unpleasant. On heroic mode you have no room for error and a slight delay is certainly fatal instead of just really bad.
People noticed this of course and developed an ingenious and hilarious workaround. There is a mod that places a circular spot on your screen that you cannot click through. The idea is you place that circular spot over the gong and click furiously on it when the Searing Flame cast is coming and the mod will remove the spot the instant Searing Flame begins. Thus you should have roughly a .1 second response time to the cast and it should be interrupted every time. Hardmode raiding is absolutely full of 'do this right now or die' moments, probably epitomized by Heroic Chimeraon, but those actions simply should not require reactions in less than .5 second time windows. Blizzard noticed the huge effect the mod was having on the success of raids and changed the Searing Flame mechanic to a guaranteed tick at the start of the cast and then new ticks every 2 seconds. This means that players have a really reasonable time period to interrupt that can be hit every time under normal latency conditions. The new mechanic forces you to pay attention and play right but skilled players with normal connections will take precisely 1 tick every time and win, which is exactly as it should be. Unfortunately Atramedes already lent itself very strongly to another mod which allowed everyone to see each other's sound levels. Because this is simply not necessary on heroic (if you get too much sound you just die) I am a little less worried about it, but it is somewhat silly that there are gongs to hit to get people's sound down and yet there is no way to know what someone else's sound is without using a mod.
I am all for challenging encounters but I think the types of challenges involved need to be ones that do not so easily lend themselves to mods rather than intelligent decisions. Any time you create a challenge that requires proper allocation of resources or strategic alterations then players must respond appropriately. Challenges that are simply reaction time checks really have a minimum time window where they can be dealt with by players rather than machines and keeping all checks like that above that minimum is important so as to not make running specific mods for each encounter necessary. The casts on Maloriak (which we just downed on heroic!) are a good example of where this is challenging but doable. Once the cast time slow is applied there is sufficient time for a skilled raider to figure out which spell is being cast and interrupt it in time. It isn't perfect, of course, because having a boss mod give a 'shake and rumble' when the cast you care about starts is very powerful. That said, it is entirely doable without.