Can definitely relate to this, particularly when I'm working on feature x but it looks like feature y is almost certainly going to be required at some point.
It just seems so much easier to include the ground work for feature y while writing feature x. Then by the time it's clear how much extra time it will take to debug and test those bits of groundwork, it's too late to return to a focussed implementation of feature x.
I don't have problem with debugging and testing - since I'm programming in Ada most of the things that compile are already working properly (modulo errors in logic of program, but nothing in reality can help with this). In fact the only place where I just need to do serious testing is in UI and input reading (who knows with what someone may try to feed your program).
But this demands some designing before programming - and then you are seeing what additional features would be nice to have... and you get my version of feature creep, where you have so many features you "have" to write that you don't have time to release version alpha.
I definitely tend to suffer from the second type as well, lucky enough to have a great product manager who keeps the specs tight and limits how much I can "just add this one really cool thing!"
It just seems so much easier to include the ground work for feature y while writing feature x. Then by the time it's clear how much extra time it will take to debug and test those bits of groundwork, it's too late to return to a focussed implementation of feature x.