Adhesive bonding is applied by the aircraft industry both for assembling composite structural parts and implementing composite patch repairs in damaged structural parts. In both applications, there exist several scenarios, related to surface contamination and processing, that could affect bonding quality and thus, degrade bond strength. In this paper, the detailed effects of pre-bond contamination with a hydraulic fluid, thermal degradation of the composite substrate, as well as poor curing (lower curing temperature) on strength of composite-bonded joints were studied experimentally by conducting mode I fracture toughness tests on double-cantilever beam specimens. These three application scenarios are possible to appear in the implementation of a composite patch repair in a damaged composite structural part. The experimental results showed a contradictory effect as the presence of the hydraulic fluid and poor curing degrades the fracture toughness whereas thermal degradation enhances fracture toughness of the composite-bonded joints. These findings are explained by means of extended non-destructive inspection, surface analysis, and evaluation of fracture surfaces.