What I was doing when I found the issue:

Editing a group activity where the initial location was set using the "Current Location" option, or when setting a new location using "Current Location"

What should have happened was:

Successfully updating the Group Activity record

What actually happened was:

ELMAH error message

type="System.FormatException" message="Input string was not in a correct format." source="mscorlib" detail="System.FormatException: Input string was not in a correct format. at System.Number.StringToNumber(String str, NumberStyles options, NumberBuffer& number, NumberFormatInfo info, Boolean parseDecimal) at System.Number.ParseInt32(String s, NumberStyles style, NumberFormatInfo info)