I encountered a particularly nasty bug or effect and tracking it down and correcting it is going to be extremely annoying.

I needed to send an exact amount of bitcoins and bitcents to a particular address. I input the address into Blockchain.info's search and brought up the destination wallet. Then I clicked Request Payment and input via copy/paste the exact amount of bitcoins and bitcents to send. The QR code changed and I scanned it with the iPhone Blockchain app.

I verified it was sending to the destination address the exact amount of bitcoins and bitcents (YYY.YYY70658). Then I clicked send payment. It actually sent (YYY.YYY707) to the destination address. It rounded up .00000042. My initial thought was the Blockchain app rounded it but perhaps there is something a little deeper at play. For example, when I have the two different wallet send amounts in Google Docs and subtract the actual sent from the proposed sent then Google Docs thinks the difference in the amounts is 0.0000004199999 repeating and not .00000042.

I thought it may have been an error with the display of the wallet amounts but verified it on blockexplorer.com also. It did send the wrong amount contrary to what was displayed on the screen before send payment was clicked.

I intended to send it from one of the two active addresses in the wallet but inadvertently left the default app selected to Any Address. Nevertheless, even if that were a contributing factor by pulling inputs  from multiple addresses the problem is in rounding up the amount sent. That output amount to the destination address should not be changed. Additionally, there was no verification of inputs and outputs like the browser based.

Suggestions:
1. Perhaps the app and web browser should display all 8 bitcents for all amounts. This rounding is problematic for usability.

2. Perhaps there should be an input/output verification with the iPhone app?