There must have been something in the air yesterday, I did exactly the same thing! Working quickly under pressure on a production system I ran an update but..... didn't highlight the last line of the where clause.....849 lines updated instead of 2...aaargh! Fortunately I preceded the update with a 'begin tran' so I rolled back and tried again!
I just reckon this kind of human error is inevitable. Steps to minimise it are (i) implement good change control procedures and adhere to them (what I did, needless to say, contravened all my employer's procedures which made it even worse for me) (ii) get into the habit of preceding all DML statements with 'begin tran' or (iii) set implicit transactions on your session or at a Server level or (iv) use the Enterprise Manager grid display to update individual attributes.
Option (ii) - get into the habit of preceding DMLs with 'begin tran' - is the easiest and most hassle free IMHO.