Well I'm still early along this road, and there have indeed been hiccups. And I'd love to hear reassuring stories from others about hiccups they experienced and then recovered from! :)
In our case, although I'd been reading ERE, YMOYL and others for many years, I got full on the MMM bandwagon a couple of years ago now. We earn what feels like decent money, and I started setting savings goals, monthly budgets, and dreaming of our soon-to-be retirement years. But over the last couple of years, although we've managed to save a little, we've also been derailed by major dental work ($6500), a month on half salary for me last summer, an upcoming paid leave at 70% salary, and this past month DH had an episode with his car that will likely end up costing us another $1000 of unexpected cash. Then it turned out I wasn't budgeting nearly enough for our home maintenance fund, we're going to need to replace the car a little sooner than planned, some other more minor issues came up, etc etc. You know, Life Happens. But we just can't seem to keep an emergency fund intact.
It's frustrating, because generally we're on track, and when I run the long-term numbers, everything still works out fine. But in the short term, I struggle, because we're THIS CLOSE to getting ahead and really building some savings, and we just can't quite seem to get there. My sister points out that our financial skills mean that we can absorb these costs, and our debts aren't mounting, they're still shrinking. Sigh. I remain optimistic, and trust that YNAB and our daily frugal habits will eventually win out. :)