I must confess to being confused by the whole of the responses now. Because it seems like some are saying it is legal (or at least isn't illegal) to run over the catcher.

It's my understanding that MLB (and maybe NCAA) allows a runner to crash and dislodge the ball from a catcher, but doesn't allow a runner to crash and dislodge the ball from the other infielders.

Assuming that is the case, then there is a discrepancy.

And the discrepancy is either accounted for in the rules (i.e., the rulebook makes an exception for intentional collisions at the plate), or the discrepancy is a result of an "unofficial" rule.

It seems as though the book in MLB and NCAA prohibits crashing into any defender. Correct? But that "unofficially," this isn't ever called at the plate.

(Note: this is simple curiosity from a baseball fan and a football/softball/lacrosse official. I'm not criticizing anyone!;))

MLB (and MiLB too) has no malicious contact rule. NCAA and FED do, so they're different.

OBR thus does not prohibit crashing any fielder at any base. You're right to observe that you don't see it at other bases: at 1B, of course, it wouldn't make sense.

At 2B and 3B you don't see it because crashing a fielder doesn't make baserunning sense: on a force play it does no good, and on a tag play you're likely to end up off the base and tagged out anyway for the trouble.

It's also against baserunning tradition, and as you point out there's an unwritten rule against it. That's a rule that the players enforce among themselves: the umpires do not enforce it.

The existence of unwritten rules and enforcement (usually a fastball in the ribs) is part of what led amateur ball to add a MC rule including a penalty enforce by umpires.

Some of the modified OBR games around here do have a malicious piece that is enforced by the umpires. There's no point in the adult league rule set that has the word malicious in the rule set. It simply says "pro slide rule will be used at 1B,2B,&3B. College Slide will be used at home." I wish they'd use the proper terminology though. Without a malicious element in there, we have to interpret straight OBR guidelines.

Your understanding is incorrect.

A crash will rarely take place at a base other than home plate for the simple reason that the runner is not in jeopardy if they tag an overrun home plate.

If a crash take place at second base, there's a pretty good chance that you are going to have MC on the play.

Or possible willful and deliberate interference w/ the intent to break up a double play if playing OBR