The problem with the interface you propose is how exactly Fiddler would identify a "similar" session-- there are an infinite number of criteria you could choose.
Fortunately, you can specify such criteria yourself.
E.g. Click Rules > Customize Rules. Scroll to OnBeforeRequest
oSession["ui-backcolor"] = "yellow";
oSession["ui-color"] = "#000000";
Save the file and all future requests whose URL contains "error.gif" will show in black on yellow. You can obviously customize the colors and identification criteria to taste.
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