Option _Explicit Keyword(s) of day XXX:

[Kernelpanic]

@Terry -  Correct. If OPTION _EXPLICIT were the first line of code in the example, the variable mm would have been flagged immediately. Also, a constant is not an ordinary variable. A constant can't have its value changed once set. The IDE will flag this if you try it:

Ordinary variable is probably worded a bit imprecisely, but by now one should know what I mean: Without Option Explicit there is no guarantee that a constant variable will be changed by a typo. This can really be a very nasty bug in a larger program - until you find it.