I received a brand new Mac Book Pro, did very little on it. Brought it up to date with Software Update, installed Microsoft Office 2011 for Mac and brought that up to date with Microsoft also.
I connected Mac Mail to the work Exchange webmail service and started using the computer.
I noticed in some html emails, one particular font had been replaced with ‘Block A’ Characters. ie, instead of letters, everything appeared as a capital ‘A’ with a box around it. This was pretty strange as I’d done very little to the computer.
I launched ‘Font Book’ which reported a bunch of duplicate fonts which I choose to automatically resolve. Re-launched Mac Mail and the problem was fixed.
I can’t be sure, but I suspect installing and updating Microsoft Office 2011 for Mac installed the duplicate fonts and caused the font problem.
I’ve also seen this reported as a problem for Safari here: