Your application might be the most secure application ever written, but unbeknownst to you, the operating system is unintentionally working against your security. I’ve tested many applications that were otherwise securely written, but leaked clear text copies of confidential information into the operating system’s caches. From web caches that store web page data, to keyboard caches that store everything you type, much of the information that goes through the device can be recovered from cached copies on disk, regardless of how strong your encryption of the original files was.
In addition to forensic trace data, you might also be surprised to find that deleted data can still be carved out of the device. Apple has made some significant improvements to its encrypted filesystem, where each file now has its own encryption key. Making a file unrecoverable is as easy as destroying the key. Unfortunately for developers, traces of these keys can still be recovered, allowing the files they decrypt to be recovered.