Other things to avoid

Other things to avoid, not previously mentioned or deserving further emphasis:

I once had a file with gene expression microarray data where the gene identifiers were long integers. It had gone through Excel at some point, and the identifier 1000000 had been changed to 1e6, which of course didn’t match what was in the file with gene annotations.
I can’t stand the “Split” window in Excel (from the menu bar, Window → Split). A lot of my collaborators seem to use it, but it drives me batty. To get rid of it, use Window → Remove Split. On the other hand, I like “Freeze Panes”. Go to cell B2 and click Window → Freeze Panes. Then when you scroll down or to the right, the values in the top row (which contain the column names) and the first column (which often contain individual identifiers) remain shown.
The Data Carpentry lesson on using spreadsheets points to a common mistake of not filling in zeroes: that one might enter only the non-zero values and leave the cells-that-would-be-zero blank. Don’t do that! Zeros are data, and you’ll need them.
Spreadsheet software can have limits on the number of rows or columns, which can lead to data files being truncated. For example, the older versions of Excel (xls files) had a maximum of about 65k rows, and its use apparently led to a loss of COVID-19 results at Public Health England. The newer versions of Excel (xlsx files) are still limited in size, to about one million rows and 16k columns.