Rule #1 of Beeminder: Things that make staying on the yellow brick road  easier make reaching your overall goal harder. There’s no free lunch. Any leniency today will get paid for down the (wait for it) Road.
If that sounds counter-intuitive, imagine the extreme case of ultimate leniency where we’re like “don’t worry, honey, as long as you’re at your goal weight a year from now all deviations from the yellow brick road will be forgiven.” That completely destroys all the benefit Beeminder has to offer! 
“At some point you’re going to end up pushing the limits, which means you have to understand the precise limits”
In general, we need to minimize the number of rules and caveats to think about. Any rules and caveats that make staying on track harder are bad for obvious reasons. But even the ones that try to make it easier require you to think about them. That’s because at some point you’re going to end up pushing the limits, which means you have to understand the precise limits. Consider grace periods, three-strikes policies, and generous exemption criteria. Those, ironically, do not reduce your chances of derailing!
How not? Because they don’t make the overall goal any easier. They mostly just add some fuzziness about how far you can push things before you really derail. Which just means you push a little farther than you would otherwise and end up increasing your chances of derailing. Better to push it / procrastinate up to a bright, unambiguous red line than to push it / procrastinate up to a line whose exact location is shrouded.
The lesson we’ve learned (the hard way): make the goal itself easier, don’t try to make it easier with various leniencies.
Catching Back Up Just This Once
The general rule is that leniencies necessarily backfire. Sometimes newbees unaccustomed to our hard-assery will balk at a derailment that they view as still recoverable. But catching back up “just this once” is, like, defeating ze whole point of ze commitment device.  If that’s even a possibility in your head then we can predict failure with pretty high confidence. The problem with that mentality is that you may catch up once or twice, but each catch-up will take longer until it just never ends up happening. Which is why, early on, we made it fundamental to the way Beeminder works that you pay the penalty the very day that you cross the bright red line.
UPDATE: Since this post is kind of a classic that we point to often, we made some edits to modernize it slightly. Namely, we changed some of the references to the yellow brick road to instead refer to the bright red line.
Image credit: adorable plushies by Heidi Kenney. Visit her blog if you need a faceful of cute.