It appears that, like its desktop relative Mac OS X, the iPhone's OS X operating system needs some head-room on the internal flash memory in order to operate properly. We've begun receiving reports from readers indicating that iPhone's filled to the brim with data exhibit issues with crashes in applications, freezes and others.
Our sister site MacFixIt notes:
"Mac OS X requires at least 10 percent of the volume it is contained on as free space in order to maintain the integrity of the file system. However, even with 10 percent free space, Mac OS X's use swap files - as well as extra data generated by third-party application caches, etc. - can quickly put you back into a position of possible directory/file damage."
We're not sure yet how exactly the iPhone's filesystem works, but for if you're experiencing repeated problems similar to those aforementioned, try clearing up about 10 percent of its free space (by reducing the number of files/data synced to the device in iTunes) and check for alleviation.