I have had a few mechanicals in my day, and learned long ago that it is wise to be prepared.  As such, I have a small bag on all my bikes that contains the basics to get me home after a flat or other minor mechanical.  This is a view of the two kits I use on my commute bike and rando bike.  The basics are all here, and include:

1. Spare tube for tire
2. Tube patch kit (for more than one blowout)
3. Tire Irons
4. Zip Ties (cheap fasteners)
5. Tape (ditto)
6. Chain tool
7. Basic hex wrenchs
8. Phillips and flat head screwdriver
9. Seat cover for leather Brooks B17

I have a few extras in my randonneur bag:

1. Lock/Cable
2. Cargo net
3. Spare Change
4. Knife
5. Handkerchief
6. First Aid supplies/Ibuprofen
7. Latex Glove/Rag

Now if I’m on a long solo ride, or camping trip, all bets are off.  I would have more kit for more extreme rides, no doubt.  This stuff is the basic safety net I have with me all the time, however.