The Ba’al HaTurim draws our attention to ancient texts in which the first letter of tamim, the tav, is written large.
When large letters appear elsewhere in the Torah they denote emphasis and warn us against careless mistakes, e.g. the large dalet of echad in the Shema (Deut. 6:4), which may be a warning against writing the similar-looking letter resh, which would change the whole meaning from “The Lord is One” to “The Lord is another”.
So what letter might be mixed up with a tav?
A careless scribe might use a heh instead of a tav, which would change “wholehearted” to “confused” (the meaning of the root h-m-m). The result would be an impossible command that we should be all mixed up about God and uncertain about where we stand.