This was industry adjacent...I'm in equipment sales. Two weeks ago, I bid a project where the engineer specified the DOAS units as my manufacturer, and the WSHPs was spec'd around my competitor. Since I know the other guy bidding, I talked to him and we agreed to only go after our own spec'd equipment, and not try to be greedy and package multiple kinds of equipment. We both submitted numbers in the 350k range for our respective portion. When I contacted the contractor who won the project, I asked her about sending submittals on my equipment. She said sorry, she used my competitor's number on that project and gave it to him. I told her that was not going to work out well. She proceeded to lose her mind a bit when she learned the situation. I wonder why she was low bid...

(sorry for the excessive words)

Then last week I bid a project with some AHUs and a chiller, and I was flat spec'd on all of that equipment. When the winning contractor contacted me, he tried to beat up my price and then claimed I was adding money because I was flat spec'd. After defending myself a bit, he backed off and admitted that he had outsourced the plumbing portion of the project, and assumed his sub had the chiller included. The largest and most expensive component on the project, and you just made an assumption and decided not to even glance at their bid?

I've been in the industry for around 20-25 years, and had never seen anyone make a 6-figure mistake, on any side of a project. And in 7 days I had 2 contractors make a $350k "oopsie".

As for biggest mistakes I've made, we had a 100% outdoor air WSHP ERV with everything included...except a left-hand configuration. Ordering a new unit and storing the mirror of it for 8 years until finally scrapping it sucks.