Trailing earnings are audited history, which is the point. They include every one-off, so a year containing an impairment produces an artificially high trailing multiple that has nothing to do with the run rate.
Trailing measures are least useful exactly when they are most tempting: at a cyclical peak the multiple looks low because earnings are at a high they will not repeat, and at a trough it looks high for the mirror reason.
Example: Northwind Tools earns $0.81 of trailing eps and trades at $26, a 32 times trailing multiple. Excluding the $11M restructuring charge, adjusted trailing EPS is $0.90 and the multiple is 29 times.
Related: forward-pe, pe-ratio, eps, earnings-yield, peg-ratio