Alignment errors are caused by misaligned reads and writes. For example, a two byte read where the memory address is not an even multiple of two bytes is an alignment error. Alignment errors are caused by a software bug.
Troubleshooting Spurious Accesses, Alignment Errors, and Spurious Interrupts
To make it a bit more practical, take a look on this example. A server connected to a switch. This server has 100/Full and the switchport configured as 100/Full too. However, when the server check its interface, it shows that it connected to switchport 100/Half.
Another example can be seen when a device is located in distance area or connected using fiber converter. Use this command to look for interface errors.
# sh controllers ethernet-controller fastEthernet 0/1
Transmit Receive
2726272818 Bytes 2358979949 Bytes
13697375 Frames 233194251 Frames
152961 Multicast frames 823937 FCS errors
415811 Broadcast frames 6104806 Multicast frames
0 Pause frames 5721647 Broadcast frames
0 Single defer frames 0 Control frames
0 Multiple defer frames 0 Pause frames
0 1 collision frames 0 Unknown opcode frames
0 2-15 collisions 1004662 Alignment errors
0 Late collisions 0 Length out of range
0 Excessive collisions 0 Symbol error frames
0 Total collisions 0 False carrier errors
0 Control frames 0 Valid frames, too small
0 VLAN discard frames 0 Valid frames, too large
0 Too old frames 21075 Invalid frames, too small
0 Tagged frames 17609 Invalid frames, too large
0 Aborted Tx frames 0 Discarded frames
Transmit and Receive
196254216 Minimum size frames
14150753 65 to 127 byte frames
10031908 128 to 255 byte frames
7890623 256 to 511 byte frames
3075064 512 to 1023 byte frames
15450378 1024 to 1518 byte frames
0 1519 to 1522 byte frames