Only change one thing at a time, whether it’s a new practice, debugging code, communications/messaging, or culture-related. You can more easily detect the isolated feedback from that change if you alter only one thing at a time,
Although this seems fairly obvious, here are a few hints and tips:
Debugging code: first, prove the bug exists, preferably with a test written in code. Change one thing that you think will fix it, and run the test again.
Debugging process: similarly, don’t make a change unless you have some feedback showing the problem. Change one thing, and re-evaluate the feedback.
Changes to one area results in unexpected results somewhere else: that’s good information to track down; there’s a connection there you weren’t aware of. But now, how to handle this unexpected, unplanned line of investigation?
When you’re doing “just one thing,” and the unexpected pops up, how should you handle it? If it’s directly related to the “one thing” you’re working on, it’s probably best to investigate it now. If it can wait, defer it until later.
If it’s not clear, have a short, time-boxed meeting to determine a course of action. At the end of the alloted time, take some action, even if that means flipping a coin or rolling a dice to make a decision. Action is still preferable to inaction. Also, any investigation and/or correction doesn’t have to involve the entire team.
When this sort of thing happens regularly, then it’s a sign that you need to spend more regularly-scheduled time understanding the code base better.
If these unexpected surprises comes from other groups (sales, support, etc), involve your ProductOwner. They have the big picture perspective that helps them effectively triage “urgent” requests. There’s always a balance between responsiveness and getting work done. Your team needs to decide where you need to be on the sliding scale.
No, studies have repeatedly shown people are not good at multitasking at all and in fact it costs them a great deal of productivity. See, for example, these studies, or the books Pragmatic Thinking & Learning by Andy Hunt and Flow: The Psychology of Optimal Experience by Mihaly Csikszentmihalyi for more information.