Imagine you have to wait 2 minutes before you can do something you need to get done, it's otherwise wasted time so you decide to work on some trivial task, say sweeping the floor, or washing the two or three dishes in the sink.

You start washing dishes, and someone sees you doing it.  To them, you're "doing the dishes", not "filling the two minutes while you wait", so it's perfectly reasonable and helpful for them to bring dishes over to you (or start moving furniture to help you sweep every corner of the floor in every room in the house).

Now you are committed to a task which has telescoped beyond the time-frame you'd originally planned for it.  You have other higher-priority tasks you were intending to get to, but you're "in the middle" of the telescoped task.  The total time you'll spend on it is only maybe twice the time you planned, but now you are berating yourself for starting the task before doing the high-priority task, instead of congratulating yourself for slotting the task into the interstices.  The next time you'll think twice about starting the micro-project while waiting, as it may turn out to take twice as long as it appears.

Efficient use of time is often a matter of slotting the "little tasks" into the gaps between the big tasks.  But when you do this kind of time-management, you need to communicate to the people around you that you are just filling the time as you wait, not committing to complete every aspect of the task.  Otherwise, trying to help, they will commit their time and energy to helping you get everything done as fast as possible.  Once they've committed their time to the task, you lose efficiency if you drop the task, so you wind up throwing your time-management out-of-whack.

Basically, "slotting" tasks requires that you be able to accurately guess how much time the task will take.  If a task can suddenly telescope then you can't say for sure that you can get it done in the 15 minute train ride, or the 2 minute wait in a hallway somewhere, so you have to set aside an "open ended" block of time for it (or risk having it stuck half-done, which for many personalities (mine included) sucks rocks).  Open-ended blocks are much harder things to come by than 5-minute interstices, so the more things that show up with loose estimates, the longer the "big todo" list grows.

So, in short:

• you have to communicate to those around you that you're filling time with a hard-estimate task
• they will commit their own resources to your task (because they are trying to make the task easier for you)
• their effort will potentially expand the task beyond your estimate
• their effort will often increase the per-session startup/teardown cost until you pretty much have to finish it to avoid wasting effort
• the better you can estimate a task, the more likely it is that you can slot it into an interstices
• the more you can break a task down into "hard-edged" component pieces the more likely you can slot the pieces into the interstices
• if your estimates are thrown off on a particular type of task a lot, you'll no longer "trust" that task as something you can get done while waiting for something else
• this is going to happen, try to keep in mind that the telescoped tasks are now done
• so maybe an open-ended block of time will happen in the future
• or maybe you won't have to do this particular task for another day
• your plan getting thrown off isn't really all *that* important, no matter how much it feels like you've just thrown off your schedule by *two whole minutes*
• writing blog posts about time management is a *really* bad time-management strategy
• you can spend many months worth of "saved" minutes on a single post, so just do the dishes already :)