I created a web page on which I can recreate this on my 4th-gen iPad. I went to the local Apple Store to test this on the devices there and here are the results (this store had none of the new iPads):
Devices which had the same problem
iPhone 5s
iPad Mini Retina
Devices which did NOT have the same problem
iPad Air
iPhone 6
iPhone 6 Plus
Not tested
iPhone 5c
My 4th-gen is an AT&T model. I have a 4th-gen at work which is Verizon. The work model has the same issue as my personal device.
Want to try this for yourself on your device? I copied the HTML where I had the problem, stripped a lot of stuff out, and put it on my web site. If I take any more out, the problem stops happening.
Make sure your iOS Safari is in Public mode (all my testing was in that mode), Auto-Correction is on, and Predictive is also on.
Go here : http://www.patrickjmaloneyllc.com/iOS8LockupTest
Put your cursor at the end of the "Subject" field. There is one space at the end. Make sure only the cursor is displayed after this space (make sure no spell check, etc. bubble is displayed). Now, start tapping the backspace button. Once a second, three times every two seconds, something like that. If you have the problem, Safari will lock up after deleting a few characters.
If you have the problem, please post a bug report at one of these two links or both (you might need a developer account for the first one):
http://bugreporter.apple.com
http://www.apple.com/feedback
You can include my test URL in your report. I will leave that up until the problem is corrected. Everyone should post a report so we can get this fixed.
Regards, Patrick