Is it possible to get 100% Apex code coverage all the time?

For all but the most trivial code, 100% coverage isn't achievable. Generally speaking, code coverage generally follows the Pareto principle: Covering 80% of your code only requires about 20% of the code it would take to get to 100% coverage. In most cases, the very first test method I write usually gets about 80% of the coverage (but can vary between 40% and 100%), and an additional 4-6 methods are necessary to approach 100%. Very simple code with no (or very few) branches can be covered with a single test method, but for most practical code, you'll need multiple methods to reach high coverage.

You'll generally have four categories of code:

• The main execution branch. This is covered in 1 test method, and should reach about 80% of your coverage (if not, your code probably needs refactoring).
• The side execution branches. These cover normal situations, such as showing warnings on a page, different paths of execution depending on input, etc.
• The exception branches. These cover unusual situations that have to be handled in a special way. You should not normally code to try and cover these branches, just know that they work. This code should be no more than 20% of your total code, and is usually closer to 10%.
• The impossible branches. Some code simply can't be run, because of the various limitations of test methods, including some sharing, branches that can't be reached normally but could be reached in unusual situations, and methods that can't be called without causing a "skip" message. This should be no more than 5% of your code, and is usually far less (<1%).

I personally strive for 90% coverage in all classes; this gives a safety margin of 15%. I usually write my code in ways that seem unusual, but result in better coverage through fewer lines of code.

On average, I usually get to about 95%, with the other 5% being exception code and/or impossible-to-cover code. Also, it's worth noting that the percent covered is inversely proportional to the size of the class, on average. At less than 50 lines, I'm almost certain to reach 100%, but at 5000 lines, I'll be lucky to reach 90% in any reasonable amount of time. It usually takes more time to write proper test methods than it does the code you're testing.

These are 2 off the top of my head for starters...

PageReference.getContent() and PageReference.getContentAsPDF() can't be called by code running under test.