Yep! The problem happens when you divide the safety buffer up in the first place. Safety buffers demand to be shared, when one part does not use all of its safety margin you want to transfer that to another system.
Another surprising place where this happens is project scheduling. We budget time for each individual step of a project based on our guess of a 90% or 95% success rate, then our "old-timers' experience" kicks in and we double or triple our time for all the steps together, then our boss adds 50% before giving the estimate to their boss, which sounds gratuitous but it is to protect you because their boss looks at how grotesquely long the estimate is and barks out a cut of 20%, so the overall effect of those two is (3/2) × (4/5), so your boss still netted you a 20% buffer while making the skip-level feel very productive and important.
Say the 50%-confidence-to-95%-confidence gives you 30% more time as safety buffer, and you only double the estimate, and the work that you missed in your initial assessment, while it's not gonna be say half the project, maybe generously it's a third of the project or so. So the project actually takes time 1.5 measured properly, you have together budgeted 1.3 × 2 × 1.2 = 3.12 time. The total project deadline is more than half composed of safety buffer. And we still consistently overrun~!
But if Alice needs to work on some step after Bob, and Bob finishes early, when does Alice start on it? Usually not when Bob finishes. Alice has been told that Bob has until X deadline to complete, and has scheduled herself with other tasks until X. Bob says "I got done early!" and Alice says "that's great, I'm still working on other things but I will pick my tasks up right on time." Bob's safety buffer gets wasted. This does not always cause any impact to the deadline, but it does for the important steps.
Of course, if you are a web developer you already know this intuitively because you work on servers, and you don't run your servers (Alice, for example) at 100% load, because if you do then you can't respond to new requests (Bob's completion event) with low latency. It's worth thinking about, in an efficient workplace, how much are you not working so that you have excess capacity to operate efficiently?
Have you ever thought of just accepting that you can't predict how long a project will take to complete?
It's a revelation. You get to have some hard conversations with other managers. But in the end everyone finds it easier to deal with "it'll be ready when it's ready" rather than endless missed deadlines and overruns.
In my experience people demand fantasies, and will fight tooth and nail any encroachment on them by the reality that things never happen as they are planned. Although when I say this, I am thinking of one year and above kind of estimates.
When people who are experts on the topic evaluate the work needed over a period of say 3 months, even in something as notoriously hard to plan as video game production, it can hold. This entails being willing to adjust scope and resources though, when planning, in order to ensure the objectives are likely to be met.
This never worked in 15+ years I've been working. The people who did try that went out the door very soon once management realised some other person could tell them a date and they could plan their business around that date even though that date half of the time got missed anyhow.
Yeah it's tough to get the point across. Worth it if you can, though. For everyone - no-one enjoys rescheduling everything because the deadline was missed again
Another surprising place where this happens is project scheduling. We budget time for each individual step of a project based on our guess of a 90% or 95% success rate, then our "old-timers' experience" kicks in and we double or triple our time for all the steps together, then our boss adds 50% before giving the estimate to their boss, which sounds gratuitous but it is to protect you because their boss looks at how grotesquely long the estimate is and barks out a cut of 20%, so the overall effect of those two is (3/2) × (4/5), so your boss still netted you a 20% buffer while making the skip-level feel very productive and important.
Say the 50%-confidence-to-95%-confidence gives you 30% more time as safety buffer, and you only double the estimate, and the work that you missed in your initial assessment, while it's not gonna be say half the project, maybe generously it's a third of the project or so. So the project actually takes time 1.5 measured properly, you have together budgeted 1.3 × 2 × 1.2 = 3.12 time. The total project deadline is more than half composed of safety buffer. And we still consistently overrun~!
But if Alice needs to work on some step after Bob, and Bob finishes early, when does Alice start on it? Usually not when Bob finishes. Alice has been told that Bob has until X deadline to complete, and has scheduled herself with other tasks until X. Bob says "I got done early!" and Alice says "that's great, I'm still working on other things but I will pick my tasks up right on time." Bob's safety buffer gets wasted. This does not always cause any impact to the deadline, but it does for the important steps.
Of course, if you are a web developer you already know this intuitively because you work on servers, and you don't run your servers (Alice, for example) at 100% load, because if you do then you can't respond to new requests (Bob's completion event) with low latency. It's worth thinking about, in an efficient workplace, how much are you not working so that you have excess capacity to operate efficiently?