不 with rising tone

I was listening to Pimsleur and 不客气 was explained as a rising, and two falling tones. I thought 不 is pronounced with a falling tone, so I looked it up. This is what xuezhongwen.net says.

不客气 bu4 ke4 qi4 you're welcome / impolite / rude / blunt / don't mention it

Is this a pimsleur mistake, a dictionary mistake, or some rule that the 不 changes into a rising tone because it is followed by a falling tone?

Is this a pimsleur mistake, a dictionary mistake

or the 不 changes into a rising tone because it is followed by a falling tone?

This is actually something that occurs with a fair amount of regularity. The tones of the characters 一 and 不 are comparatively fluid. I'm no expert on these specialised sandhi, but I can say with some certainty that both tend to pick up rising tones before a falling tone. "一个" and "不是" are examples of this. Someone else might be able to better explain this. It's been a while since I actually learned this one.

To add to what others have said:

In my experience, it is uncommon for tone sandhi (i.e. when tones change based on neighboring tones) to be represented in written pinyin text. Not only 一 and 不, but also changes in third tones when followed by another third tone (i.e. 你好！ is usually represented in pinyin as ni3hao3, even though tone sandhi makes its actual pronunciation ni2hao3).

Hope that makes sense. . . .

Chinese sandhi is quite complicated indeed. For some Chinese learners, to master the sandhi is crucial if they want to speak perfect Chinese after knowing how to pronounce some hard syllables.

the sandhi of 一:

1.Being used singly or at the end of a word, "一" should be read as original tone yī .

e.g.

一、二、三 统一 世界第一

2.Two kinds of sandhies

2.1 Before the first, the second and the third tones, "一" should be read as the forth tone yì.

e.g.

一天 yī tiān → yì tiān

一年 yī nián → yì nián

一碗（米）yī wǎn → yì wǎn

2.2 Before the forth, "一" should be read as the second tone yí.

e.g.

一万 yī wàn → yí wàn

一夜 yī yè → yí yè

the sandhi of 不：

a)Used singly

b)at the end of a word

c)before the first, second and the third tones

at a),B),c) situations, 不 should be read as its original tone bù.

e.g.

不！bù 不相干 bù xiānggān 不好 bù hǎo 不行 bù xíng

One sandhi:

Before the forth tone, 不 should be read as the second tone bú.

e.g.

不坏 bú huài 不去 bú qù 不要 bú yào

Thanks to all. Are 一 and 不 the only characters affected, or just the most common?

In my experience, it is uncommon for tone sandhi (i.e. when tones change based on neighboring tones) to be represented in written pinyin text.

This is as it should be, because, according to the rules of Pinyin, tone sandhi is not indicated.

Just remember that Pinyin is for words, not characters, and you should probably be fine.

Are 一 and 不 the only characters affected, of just the most common?

They're not the only words/morphemes (not characters) affected, just the most common.

(edited after my brain turned itself back on)