Not the biggest of ideas but a needed one I feel. There used to be a strike through option on text which seems to have disappeared a while ago and subsequent lithium releases don't seem to have corrected this (so it doesn't seem to be a bug as such).
Problem
When you have made a mistake in a reply and want to correct it so as not to cause confusion, the only options now are to delete the reply or change it (which can be misinterpreted as trying to cover up a mistake).
Solution
The strike through option is a useful way to show the previous answer has been corrected and allow the correct information to be given. It should be easy to get it back as it used to be an option.
Example:
You get 350 inclusive mins on the £10 goodybag
Edited to this:
You get 350 inclusive mins on the £10 goodybag
Correction: that's 250 inclusive mins on the £10 goodybag.
PS. I had to use HTML to get the strike through on this (all credit to stephenmiller on showing me how 
)