This is beveled and extruded T. It's not Arial, but it shows the problem.
I have colored to green the top of the bevel ridge. If you increase the bevel height setting, the green area squeezes. When it reaches zero, you get a warning of self cutting bevel. The warning stays until you close the dialog.
If you add the effect to some writing, self-cutting happens very easily in the narrowest part. You can avoid it by keeping "preview" OFF until you have typed 1 pt or another reasonable small number to the bevel height cell. Typing alone isn't enough, you must also click the cursor to another input cell.
Some shapes are impossible to get beveled properly in Illustrator. They give always error message "self cutting bevel" and unpredictable results. Examples:
- shape with a stroke
- triangle or other shape which has a sharp sub-90 degree corner.