The periodic nature of photonic crystals (PCs) (Yablonovitch 1987 Phys. Rev. Lett. 58 2059–62; John 1987 Phys. Rev. Lett. 58 2486–9) has been extensively exploited for the past quarter of a century using photonic bandgap (PBG) effects to manipulate photons in engineered electromagnetic structures. Structures such as photonic crystal nanocavities are widely considered to be key in realizing future nanoscale optoelectronic devices. These cavities are capable of creating resonant modes with high-quality factor (Q) and small mode volume, in other words a large Purcell factor (Purcell 1946 Phys. Rev. 69 681), and have been widely researched in the two-dimensional photonic crystal slab (PCS) defect cavity configuration (Painter et al 1999 J. Opt. Soc. Am. B 16 275–85). Here, we demonstrate for the first time how three confinement mechanisms are thought to coincide to give rise to a high-Q resonance for a slab containing a modified L3 defect where the slab thickness is such that the guiding in the slab is no longer single moded (Tandaechanurat et al 2008 Opt. Express 16 448–55). This is in contrast to the conventional design approach, where the PCS thickness is chosen to be of the order of half a lattice constant to ensure that a PBG exists to confine cavity modes strongly within the slab (Painter et al 1999 J. Opt. Soc. Am. B 16 275–85; Johnson et al 1999 Phys. Rev. B 60 5751–8; Khankhoje et al 2010 Nanotechnology 21 065202). These newly identified high-Q modes can be important in terms of the fabrication of slabs and other devices such as vertical pillars since they allow high-Q factors in thicker and more fabrication tolerant geometries.