Visual Supply Co. (VSCO) develop VSCO Cam, which has quickly become my favourite photography app for iPhone. I have it permanently on my homescreen and use it for 90% of photos I take with the iPhone now.
VSCO Cam replaced a previous app by the same company called VSCOCam (yep, without the space), which I onwned and really liked. When I say replaced, I mean exaclty that. The previous version is no longer available or supported.
While the previous one was a paid app (from memory it was US$.99), the new one is free with in-app purchases. If you own the old one and have it installed, you get a bunch of paid filters for free when you download the new one. This is a nice touch from VSCO.
But the new filters are even better than the old ones. I purchased the full pack of filters and I'll be spending some quality time with them. So far, I really like it.
One thing I noticed when I bought the filter pack is that it kept giving me an error in the middle of the download. The error message said only "There was a connection error" and all I could do was start the download again. I tried multiple times and always got the same error.
Frustrated, I tried again and watched the phone through the download instead of just letting it do it's thing. That's when I realised the download took so long that the screen went dark and the iPhone locked.
If you purchase the full filter pack, it's about 108MB and I assume the app can't download in the background.
So, to fix the VSCO Cam download error issue, just go to the Settings app, then General > Auto-Lock and set it to Never. Then start the download again. It'll take a while, but it'll finish fine.
Once it finishes, remember to go back and set the Auto-Lock to whatever you had before.