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only sometimes after. it only happens in safari after i've been using it awhile. maybe it has to do with a shortage of ram or maybe the iphone's doing one too many things at once? i don't know. try restarting it, and see if it speeds things up.
 
Restarting it with holding down the sleep/wake button and home key works for me to fix this issue. Or when its actually happening just hold the Home key down for 6-8 seconds and it will force quit.
 
My roommate had it happen to him once in Safari. I haven't encountered it anywhere else though.

Again, probably just a software bug that hopefully Apple will fix.
 
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