Here's a shot of a more or less correct approach to Liverpool Airport (lined up a little outside):
This is in clear daylight VFR conditions. Notice the taxiway to the right; long, straight, flat, and very runway-like. If the conditions weren't as clear and you were in a pattern where you crossed by the taxiway first, you might mistake it for the runway.
Here's a circling shot of DFW International from the southeast, almost in line with 31R to the right of the picture:
Each runway has a full-length or near-full-length taxiway, and because of the heat, movement count and weight considerations of the planes it regularly sees, all surfaces at the airport are concrete to minimize maintenance and resurfacing needs. Even on the normal north-south approaches, it can be difficult in some VFR conditions (with no approach lighting) to distinguish the runways from adjacent taxiways. The outlying diagonal 13/31 runways are worse in some regards as their taxiways are full-length and nearly as wide as the runway itself, and they have, in some circumstances, been used as runway surfaces, so the taxiway has a few tire scuffs and jet blast marks at the expected places for a runway. If you can't clearly see the threshold marker due to glare, it's not hard to at least line up for the taxiway and have to go around once you realize your mistake closer in.
In general, landing on a taxiway is more likely to happen:
- When the pilot is unfamiliar with airport layout, including student flights, diversions and recent renovations
- When the taxiway looks similar to the runway, i.e. same surface material and similar length/width
- In IMC at smaller D/E airports with poor/grandfathered lighting
- When the taxiway and runway layout or materials are opposite pilot expectations (e.g. taxiway outside the runway relative to primary terminal/hangar facilities)
- When the taxiway is the runway, or has been recently or regularly (during maintenance of the usual runway surface, or when abnormal aircraft movement counts require additional runways, like Oshkosh during EAA AirVenture)
- When a low-altitude, shallow glide slope approach is required which reduces the aspect angle and thus the ability to identify critical runway markings on long finals
Mistaken identity is generally less likely:
- In clear nighttime conditions when approach lighting is clearly visible (the runway will be unmistakably bordered in white with standard approach lighting including strobe arrays and green end lights, while most taxiways will be bordered in blue with minimal additional lighting and not easily visible from the air)
- When taxiways are clearly differentiable from their adjacent runways in length, width, material and pathway (other answers mentioned deliberate "chicanes" added to taxiways so it's obvious from the air that's not a landing surface; taxiways intentionally running at an angle to offer a different compass heading on approach are another option, but rare as this requires increased land area)
- When the pilot is well-educated on, and/or personally familiar with, the airport's runway/taxiway layout
- When a normal and/or steep glide slope is recommended, thus giving the pilot a high-aspect look at the complex.