What do you think is the biggest mistake leaders make during their mid-tenure (approx 2-5 years in their roles)?

If the company is doing well and in a growth phase , there is room to grow and drive the growth phase.
In a more static and downside environment, the leader may be reluctant to make substantial change that they have put into place .

Losing the "fresh eyes" and being ok with it. Getting too comfortable with what is "working" and stop asking questions and listening. 

Become "native" and complacent. 

Trying to be 10x hero! Change after Change. 

At mid-tenure (2-5 years), leaders seem to settle and start coasting. They may still show improvement YoY but won’t be at the same pace as the initial years. I recommend taking on a new challenge within the company or changing the paradigm of existing work to have induce fresh thinking.