In fairness, some members of the congregation have been lulled to sleep by the smooth, monotone of the priest as he/she blesses the bread and wine. Out of habit, that word just slips out. If this has happened to you, you may recall the disapproving glances and rampant judging by your fellow pew mates. You can either turn beet red, laugh self-consciously, run screaming out of the church until Easter, or...let me help you.
You see, out of pastoral concern for my fellow Christians (it is Ash Wednesday after all), I've come up with a short list of words you can say if you catch yourself in time. Once you realize you're the only one who has vigorously proclaimed "Allelu..." you can add these ending to change direction and save face. Here goes.
Allelu...theran (works better if you actually are one)
Allelu...re me in but don't take advantage of me
The only one I'd caution you against would be Allelu...cifer. That might get you in trouble with the priest.