indy91 posted an image copy of a page from Michael Collin's CMP Solo Book. The text contents of that image are
APOLLO 11 FLIGHT DATA FILE CMP SOLO BOOK
RESCUE INDEX
CASE 1 - PARTIAL DOI ( <25 FPS) (CSM ACTIVE)
CASE 2 - PARTIAL DOI (>25 FPS) (CSM ACTIVE)
CASE 3 - NO PDI1, +12 (LM ACTIVE)
CASE 4 - <60 NO PDI1, +12 (CSM ACTIVE)
CASE 5 - ≥60 NO PDI1, +12 (CSM ACTIVE)
CASE 6 - PDI1, <10 VARIABLE INSERTION (LM ACTIVE)
CASE 7 - PDI1, +12 (10-12.5 MINUTES) (CSM ACTIVE)
CASE 8 - PDI1, +14:12 (12.5-15 MINUTES) (CSM ACTIVE)
CASE 9 - PDI1, +21:24 PREFERRED LIFT-OFF(T2) (CSM ACTIVE)
CASE 10 - NO PDI2, +12 (LM ACTIVE)
CASE 11 - <40 NO PDI2 +12 (CSM ACTIVE)
CASE 12 - 40 - 90 NO PDI2, +12 (CSM ACTIVE)
CASE 13 - ≥90 NO PDI2, +12 (CSM ACTIVE)
CASE 14 - PDI2 <14:30 VARIABLE INSERTION (LM ACTIVE)
CASE 15 - PDI2 +19:22 PREFERRED LIFT-OFF (T2) (CSM ACTIVE)
CASE 16 - CONTINGENCY INSERTION ORBITS (CSM ACTIVE)
CASE 17 - MANUAL INSERTION (CSM ACTIVE)
CASE 18 - ANYTIME LIFT-OFF (CSM ACTIVE)
These eighteen cases can be distinguished by the contingency that required a rescue mission to be performed, and which vehicle is the active1 vehicle in the rendezvous. Regarding which would have made Michael Collins apprehensive, I suspect that included those cases in which
- The Lunar Module and/or the Command/Service Module had a chance of crashing into the Moon,
- Orbital eccentricity made the Clohessy-Wiltshire approximations used for proximity operations invalid,
- The computer on the LM failed, requiring the LM crew to perform the liftoff or orbit insertion manually, or
- One or both of the astronauts on the LM had a serious medical issue.
I'm surprised Michael Collins didn't say all of them made him apprehensive, but then again I am nowhere close to having the nerves of steel required of an astronaut.
1Modern nomenclature uses "chaser" instead of "active", and "target" instead of "passive".