"Help! I'm stuck on this bumper!"
"What's that bumper-sticker supposed to mean?"
For the anti-Christian: 
"If you can read this long, wordy bumper-sticker (a phrase referring to the stickers with amusing phrases people put on their bumpers), then either we're at a red light, or we're in a traffic jam."
"Stop reading me and pay attention to your driving."
"Want to know how to keep an idiot busy? (See front of car)"
"The object of the game is to forget the game. You just lost."
"This bumper intentionally left blank"
"All bumper-stickers should be eliminated"
"I need more bumper-stickers" (for those who have lots)
"This bumper is not for bumping"
"Caution: Running into cars can cause serious injury."
Bumper stickers with names of various car models, all placed near the name of the actual model, to confuse people
"[Insert amusing phrase here]"
"Are you there yet?"
"Copyright © 2008. This car may not be copied." [I don't think I was even thinking of the "You wouldn't steal/download a car" thing, maybe not even aware...]
"End confusing bumperjfqwzlxs"
"What's that? ———→"
"How's my bumper-sticker writing?"
"Carus transportus"
"Buy my bumper-stickers"
"This car has been modified from the original version. It has been formatted to fit your screen"
"Press B to start car"
"We are not responsible for driving accidents caused by reading bumper-stickers"
"This bumper-sticker is two words long." [In the original version I consistently wrote "bumpersticker" as one word with no hyphen, and when I put it on this website, spell-check didn't like that; I think my decision to use a hyphen here was to keep this one accurate.]
"The following bumper-sticker is true."
"The previous bumper-sticker is false."
"Vote 
Yes on gravity"
"What part of {x|x ∉ x} ∈ {x|x ∉ x} do you not understand?"
"
Caution: reading bumper-stickers (like this one) can cause traffic accidents."
"This bumper-sticker is not funny."
"This bumper-sticker will probably confuse everyone, but I'm putting it on anyways:"
"Vote YES"
"This car has too many bumper-stickers"
"This car doesn't have enough bumper-stickers"
"Honk if you like silence"
"Vote YES on anti-bumper-sticker legislation"
"Who put this sticker on my bumper"
"<car>" "</car>"
"My other car is a Scheme function" (or Lisp)
"All bumper-stickers are false."
"Honk if you like honking."
"Ever have a feeling of deja vu?" (mass-produce these)
"What's this sticky thing doing on my bumper?"
"This bumper-sticker is taking up valuable bumper space."
" practice = perfect"
"Meaning of my vanity plate: ________________"
"The only constant in life is change... and π... and e... and c..."
This bumper is environmentally friendly.
Please put a bumper-sticker on here that is more amusing than this one.
George Washington for President
This car has way too many bumper-stickers
Who knew bumpers were so sticky?
10 LET IT = BE
20 GOTO 10
"One man + one woman: Republicans against happy marriage"
"Don't drink and be driven: Cars against ethanol fuel." [I'm not actually opposed to it, just think that it's weird that they chose the same alcohol that's banned for drivers.]
"My {sexual orientation|gender identity} is a personal, not a political issue. LGBT+'s against politicization of LGBT+ issues."