Imagine a windward leg that is either more of a reach or due to a wind shift has become a reach and competitors are still approaching it on opposite tacks..  It would no longer be considered a "beat to windward", so 18.2(a)1 would not apply, but 18.2(a)2 would still cover the situation.

Its also not unknown for boats to misjudge or mess up a spinnaker take down so badly as to have to tack up to the mark. 

Jim and/or Chris ... how about putting up a couple drawings to make it clearer for Nick?

Simple example is a wing mark, where one boat has fallen below the reach course and has to tack back to the mark.

I don't have a diagram handy, but here are some notes I have:
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Nicolas, the difference between 18.1(a)(1) and 18.1(a)(2) is subtle but important, and it comes down to covering different kinds of opposite-tack encounters near marks, especially when wind shifts shake things up.

Rule 18.1(a)(1) is for classic windward beat situations: It says Rule 18 doesn't apply when boats are on opposite tacks and both are on a beat to windward. That's pretty straightforward; it's the typical situation when two boats are converging on the windward mark from opposite sides, both sailing upwind. This rule exists to avoid chaos in the zone when boats might try to claim mark-room even while still beating and tacking.

Rule 18.1(a)(2)  The "Weird Shift Clause" (as Dave Perry calls it). This rule applies when only one boat needs to tack to sail her proper course at the mark. It's not limited to boats beating to windward; it covers situations caused by wind shifts, current, or overstanding. Here's one example:

Facts:
Assuming the fleet was initially sailing upwind toward the windward mark on a beat to windward. 

Conclusions:
1. Rule 18.1(a)(1) does not apply because neither boat was on a beat to windward.
2. Rule 18.1(a)(2) applies because only one boat (P) needed to tack to sail her proper course at the mark.
3. Rule 18 does not apply between P and S.
4. Boat P is not entitled to mark-room and must keep clear of S under Rule 10 (Opposite Tacks).

In short:
1. 18.1(a)(1): Both boats beating = Rule 18 OFF
2. 18.1(a)(2): One boat has to tack, the other doesn't = Rule 18 OFF

Is it not simply.

If one boat overstands, the mark, mistake, curent , windshift. They are not on a beat as not closehauled, but the one on port still has to tack to sail a proper course, so we have the extra paragraph.

 Christopher Walmsley - I am struggling with "a beat to windward"  If I look at Case 132, Situation 3, a starboard boat has overstood, but it is still a beat to windward for them.  Say a shift happened after the start and the boats on the signal boat end laid the mark without tacking, but a pin end starter did have to tack and approached on port at the mark (i.e. crank situation 3 clockwise by 45 degrees).   In that case do we define this "beat to windward" if A) both boats were close hauled the entire leg? b) a beat to windward only if starboard had to tack at least once? c)not a beat to windward if the starboard boat actually was able to crack off toward the end of the leg?   Maybe another way to ask is "a beat to windward describe a leg of the course by the need for at least one boat to tack assuming they beat up the course in a seamanlike manner?  I understand that two fleets may approach a mark where it is a windward mark for one fleet and not for the other one....  but even in that case isn't it the fact that the leg is a beat to windward that controls the situation????  Thank You ... Dick