...just experienced this bug on my iPhone 6 iOS 8.0.2.
When a text message comes in and sits on the lockscreen, you can normally swipe to the left and it reveals a blue 'Reply' box, whereby you reply to the message, directly from the lockscreen without having to actually go into the messages app to do this.
If the message is relatively short, the above works fine, however, if the message is quite lengthy ( this happened with 2 separate messages which were over 4 lines long), when you swipe left to reveal the 'Reply' option and then tap it, you very briefly get the open message (in order to reply), but then it quickly crashes back to the lockscreen and the message has disappeared from the lockscreen, meaning you then need to unlock your phone, and go into the messages app to reply to it.
Can someone else test this on their phone to see if they get the same results?
Shall I report this bug to Apple (where is the link for this?)?