Misses can and do happen, and the best thing to do is to get over it when it happens. With our son, I was pretty hard on myself when we'd have a miss. Ironically, the tension I would feel made him tense, and we'd have even more misses. But once I would relax, we'd get back into the groove. It's best to just say to yourself, "whoops a daisy, we had a miss," and then get on with life. Don't beat yourself up. Do think back and ask yourself if your baby was telling you she needed to go.
Here are some situations where you'll probably have misses:
1.) When you are concentrating hard on something else.
2.) When you are watching TV.
3.) When the baby is teething.
4.) When your baby is sick.
5.) When your baby is going through a growth spurt. (This may be due to the fact they're nursing so much more, and you aren't taking them often enough.)
6.) When your baby is hitting a milestone (learning to roll, crawl, walk, etc.)
When you do start to have misses, it takes a little bit of trial and error to get back on track. Sometimes it just means you need to offer the potty more frequently, as is the case when they're nursing/eating more. Other times you'll need to back off and offer less: babies do develop the ability to start "holding it" longer, and by offering too often, you'll frustrate them. Other times they stop signaling the way they have in the past and have come up with a new signal. Always be on the lookout for new signs! Sometimes it just takes a little detective work.