Excel Macro models are stable work horses. Yet, errors do happen. Some of the most frequently occurring reasons that we have come across are listed here.

1. 32-bit vs. 64-bit: If you switch between 32-bit and 64-bit Excel, some macros may fail due to differences in memory allocation and data types.

2. .xls vs. .xlsx File Type: Old Macros may not accept .xlsx files.

3. Hard-Coded values of Dimensions, file locations, etc.

4. Overflow Errors: Some macros may encounter overflow errors during execution which requires code review for potential issues with variable types or calculations.