What is the penalty for entering Mexico via the land border without obtaining an FMM?

I do not know the official penalty, I imagine officially it's deportation or along those lines. However I do know people who have been asked to present FMM and not had one - none were at the border, rather at military, police, or specific INM (Mexico Immigration) checkpoints.

So, speaking from separate second-hand experiences:

1. Get an FMM - You are instructed to obtain one within n hours (I don't recall - less than a day)

2. Expulsion from Mexico - You are taken to the border and forced to leave the country. Doesn't seem to be any official process to this, so I wouldn't call it deportation.

None seemed to have anything official noted (that they're aware of), nothing in passports etc., and there was no problem re-entering and gaining FMM at the border.

However there is regular talk about how many US citizens live in Mexico illegally (I guess since it's often mentioned how many Mexicans live in the US illegally), so occasionally we see a lot of police and INM around, typically stopping those with current US plates. I have been asked for my Mexico Residency ID more often at checkpoints and "random" stops than I ever have crossing the border. Once they see that I have residency they wave me on, so I'm unsure how the conversation goes after that.

I still haven't found an official source, however I did find this on the Mexpro Mexico travel blog site:

Consequences of Not Having an FMM If you do not get an FMM, any of the following may happen:

• You may be detained and then told you need to go back to the border and get one.
• Depending upon your insurer, it’s possible that your vehicle insurance will not cover you if you are in an accident. Check your policy.
• If you need to be evacuated by air due to a medical emergency, you will have difficulty as an FMM must be presented for air travel (even in emergencies).

Source