[Johnfisty456]

Not asking theoretically.

Actual money.

Mine was building before validating properly.

The product worked.

The market didn't care.

Cost months of work.

Curious what everyone else's most expensive lesson was.

[G Reviews]

Mine was spending a fortune producing the product and then having nothing left for actually marketing it.

[Yakless]

Starting a company with someone who was my "friend" as the lead dev, and he just kept wanting to scale, spent about $20k, and left because of one argument.