I agree, if you have a finite and reasonable number of possible errors, the puppy/colored box idea should work great as long as each different error icon is unique enough (i.e., a puppy icon and a kitten icon would be fine, but I would avoid different types of puppies...the user will remember they saw a puppy, not necessarily what type of puppy).
Other possible ideas would be to just assign a number to your error and make it flash. Flashing gives a sense of urgency and, as long as your list of typical errors is small, the user should be able to recall a flashing number. If you get into the hundreds, this might not work so well.
"The vast majority of successful major crimes against property are perpetrated by individuals abusing positions of trust." -- Lawrence Dalzell