I'll give you one in 3E (not necessarily 3.5, because it's a little too early yet).
In 3.0 D&D, Spell DC's are set by 10 + Level of Spell + attribute bonus. There was one feat in the PHB capable of boosting the Save DC, Spell Focus, and it only did it for one Spell school.
Along comes in June 2001 The Forgotten Realms setting, which introduce the new innocuous feat, Greater Spell Focus, for a +4 total bonus. A bit more powerful than +2, but still manageable.
Then, Red Wizards and Archmages enter the picture, attainable at higher levels, but still within the lifespan of most campaigns. There is this rather neat feature called Spell Power, which boosts the DCs by up to +6 for the very dedicated.
Oooh, look! Spellcasting Prodigy! +2 effective bonus to attribute for purposes of extra spells and DCs. this is another +1 bonus.
Now, the caster is flinging off spells at a save DC of +11 more than was capable in 3E. Admittedly, this requires a good bit of resources, BUT it is resources the caster is by no means put off by expending. By late 2001, casters are in combination with attribute bonus and magic items that boost attributes casting first level spells with DCs in the low 30s, and high level spells with DCs approaching 40. Compare this to just using the Players handbook, where the DC of a first level spell cast by a caster with a 24 intelligence is about an 18.
If someone doesn't think it is power creep, then why did WotC change the rules on Spellcasting DC enhancers in 3.5? It's because they vastly underestimated what bonuses to save DCs do to the overall game balance.
Though I can't name as many for 3.5, I'm sure some of the rules masters out there can quote some examples for me. Consider the series of spells in 3E which lower spell resistance for example, and monsters with SR get far weaker. Consider magic items which allow rogues to sneak attack undead, which simply BREAKS a cardinal rule on the books by changing the nature of undead themselves.
For every example, people can explain away why this or that is balanced, and I can agree. But where power creep is duly called "creep" is because it is more subtle than the above examples. It never comes from ONE source, it comes when multiple sources are used together. The Practiced Spellcaster feat? by itself, not a problem, especially because multi-classed spellcasters do need some mechanical revision. However, the first time someone combined Practiced Spellcaster and the Wild Mage PrC in the Rules forum here, there were at least two multi-page threads of people arguing why it SHOULD or SHOULDN'T work together. (I think the "shouldn't may have won, but both sides had strong arguments.)
SUMMARY: I'm not saying it's good, and I'm not saying it's bad. I'm saying it IS, because it's human nature, and people gotta sell books.