Complete available employers
Before starting a decisive main-story mission, finish the side contacts on that island.
Story progress can remove an employer. Work left now may never be available again.

Some jobs can be lost when another strand advances. Employer order therefore matters more than in newer GTA games.
GTA III missions do not come from a list. Employers are spread across the islands, some jobs start at public telephones, and the game never shows what remains unfinished.
This has an unpleasant consequence the game does not explain: **some missions can be missed.** Main-story progress can remove an employer you have not visited. That was common in 2001 but surprises players accustomed to newer games.
Order therefore matters more than driving skill. The approach below protects a run and explains the mission types that most often cause trouble.
Before starting a decisive main-story mission, finish the side contacts on that island.
Story progress can remove an employer. Work left now may never be available again.
A save before a turning point protects the run from lost missions or a mod error.
Saving at a safehouse is free. There is no reason to keep only one save.
After certain story events, gangs begin attacking; service missions and collecting become harder afterwards.
This commonly prolongs a 100% run. Collect the packages before relationships deteriorate.
Some missions start at public telephones rather than a map icon.
Ignoring them shortens the story but loses content and completion percentage.
Learn the route first and choose a stable car, not merely the highest top speed.
Liberty City has many tight bends. A fast car crashes into them; a heavier one may get through.
Remove threats before the target moves and do not block the escape route with your vehicle.
Protected characters behave poorly and walk into gunfire, so plan for it.
The enemies' powerful weapons destroy vehicles quickly; use distance and cover.
The Cartel has the best weapons in the game. Close-range firefights usually end badly.
Damage to the mission vehicle often causes failure, sometimes only when you reach the employer.
This is the game's most common avoidable failure. Drive much more slowly than seems necessary.
Map-wide tasks with short time limits test local knowledge more than driving skill.
It helps to leave them until all three islands are open.
A missing employer usually means you advanced the main story before reaching them.
The only solution is an older save, hence the need for several slots.
Mods that change the game script can break a run midway through.
Do not change major mods or the game version during a 100% run.
The game keeps detailed counters, which reveal a missing item faster than guesswork.
The overall percentage does not identify what is missing; the individual counters do.
Answers to the main questions about this topic
Yes. Main-story progress can remove an employer you have not visited, and they cannot be restored. Complete available side contacts before decisive missions and keep several saves.
The exact number depends on whether public telephone jobs, service missions and side activities are included. The in-game statistics are more useful than one total, especially because some content can be missed permanently.
Usually because the vehicle is damaged. Many missions require the car to be in suitable condition, but only reveal this at the employer. Drive much more carefully than usual.
Some story events permanently worsen relations with a faction. It is a consequence of progress, not a bug, so collecting and service missions are best completed earlier.
Check the individual counters in the game statistics. The overall percentage does not identify the missing requirement, but the separate lines do.